Applications of First Order Differential Equations
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Questions and Answers

What is the slope of the tangent at point P(x, y) on a curve?

dy/dx

What is the equation of the tangent at point P(x, y) on a curve?

Y - y = (dy/dx)(X - x)

What is the x-intercept of the tangent at point P(x, y) on a curve?

x - y(dy/dx)

What is the equation of the normal at point P(x, y) on a curve?

<p>Y - y = (-dx/dy)(X - x)</p> Signup and view all the answers

What is the length of the tangent at point P(x, y) on a curve?

<p>y√(1 + (dx/dy)²)</p> Signup and view all the answers

What is the length of the normal at point P(x, y) on a curve?

<p>y√(1 + (dy/dx)²)</p> Signup and view all the answers

What is the differential of the area under a curve?

<p>ydx or xdy</p> Signup and view all the answers

What is the radius of curvature at point P(x, y) on a curve?

<p>(1 + (dy/dx)²)³/² / (d²y/dx²)</p> Signup and view all the answers

In polar coordinates, what is the tangent of the angle between the radius vector and the tangent line?

<p>rdθ/dr</p> Signup and view all the answers

In polar coordinates, what is the length of the sub-tangent?

<p>r²dθ/dr</p> Signup and view all the answers

In polar coordinates, what is the length of the sub-normal?

<p>dr/dθ</p> Signup and view all the answers

The curve in which the portion of the tangent included between the coordinate axes is bisected at the point of contact is a rectangular hyperbola.

<p>True (A)</p> Signup and view all the answers

What is the equation of the curve for which the normal makes equal angles with the radius vector and the initial line?

<p>r = a(1 - cos θ)</p> Signup and view all the answers

What is the shape of a reflector that reflects light coming from a fixed source into parallel rays?

<p>a parabola</p> Signup and view all the answers

What is the equation of a parabola?

<p>y² = 2cx + c²</p> Signup and view all the answers

What is the differential equation of the orthogonal trajectory of the family of curves xy = c?

<p>x² - y² = c'</p> Signup and view all the answers

What is the equation of the orthogonal trajectory of the family of confocal conics x²/(α² + λ) + y²/(α² + λ) = 1?

<p>x² + y² = 2a² log x + c</p> Signup and view all the answers

What is the equation of the family of confocal and coaxial parabolas?

<p>y² = 4a(x + a)</p> Signup and view all the answers

A system of confocal and coaxial parabolas is self-orthogonal.

<p>True (A)</p> Signup and view all the answers

In polar coordinates, what is the equation of the orthogonal trajectory for a family of cardioids r = a (1 - cos θ)?

<p>r = a'(1 + cos θ)</p> Signup and view all the answers

What is the equation of the orthogonal trajectory for the family of curves rn = a sin nθ?

<p>rn = b cos nθ</p> Signup and view all the answers

What is the formula for the velocity of a body moving along a curve?

<p>v = ds/dt</p> Signup and view all the answers

What is the formula for the acceleration of a body moving along a curve?

<p>a = d²s/dt²</p> Signup and view all the answers

What is Hooke's Law?

<p>T = λε/l or T = ke</p> Signup and view all the answers

What is the equation of motion for a body of mass m moving under gravity with air resistance kv²?

<p>mv(dv/dx) = mg - kv²</p> Signup and view all the answers

What is the formula for velocity of escape from the earth?

<p>v = √(2gR)</p> Signup and view all the answers

What is the equation of the free surface of water in a cylindrical tank rotating with an angular velocity ω about its axis?

<p>y = (ω²/2g)x² + h</p> Signup and view all the answers

What is the formula for the velocity of water flowing through a small hole?

<p>v = 0.6√(2gy)</p> Signup and view all the answers

What is the formula for atmospheric pressure p at a height z above sea level when the temperature is constant?

<p>p = poe-gz/k</p> Signup and view all the answers

In what way is the temperature of a body changing according to Newton's law of cooling?

<p>The rate of change of temperature is proportional to the difference between the temperature of the body and the temperature of the surrounding medium.</p> Signup and view all the answers

What is the formula for the quantity of heat q flowing across a slab of area a and thickness dx?

<p>q = − kadT/dx</p> Signup and view all the answers

What is the formula for the rate of decay of radio-active material?

<p>du/dt = -ku</p> Signup and view all the answers

What is the formula for the current i in an R-L series circuit?

<p>i = (E/R)(1 - e-Rt/L)</p> Signup and view all the answers

What is the formula for the rate of change of salt content (u) in a tank?

<p>du/dt = 4 − 2u/50</p> Signup and view all the answers

What is the formula for the time (t) it takes for the salt content to change from u = 40 to u = 80 in a tank?

<p>t = 25 log(100/(100-u))</p> Signup and view all the answers

Which characteristic defines linear differential equations?

<p>The dependent variable and its derivatives occur only in the first degree. (D)</p> Signup and view all the answers

What form does a linear differential equation with constant coefficients take?

<p>It is expressed in terms of derivatives and constants. (D)</p> Signup and view all the answers

Which of the following methods can be used to find a particular integral?

<p>Method of variation of parameters (D)</p> Signup and view all the answers

What is a property of the solutions of a linear differential equation?

<p>Any linear combination of solutions is also a solution. (C)</p> Signup and view all the answers

In the standard form of a linear differential equation, what do the functions $p_1, p_2, ..., p_n$ depend on?

<p>Only the independent variable. (A)</p> Signup and view all the answers

What is the significance of constant coefficients in linear differential equations?

<p>They simplify the nature of the solutions. (B)</p> Signup and view all the answers

What type of equations do Cauchy’s and Legendre’s belong to?

<p>Linear differential equations (A)</p> Signup and view all the answers

Which of the following is true about the solutions of simultaneous linear equations with constant coefficients?

<p>They may have infinitely many solutions. (C)</p> Signup and view all the answers

What is the first step in finding the complementary function for a differential equation?

<p>Writing the auxiliary equation and solving for D (D)</p> Signup and view all the answers

In solving for the roots of the auxiliary equation, which case represents two real and equal roots?

<p>(c1 + c2 x)em1 x + c3 em3 x + ... (A)</p> Signup and view all the answers

What form is used for a pair of imaginary roots in the solution?

<p>eax(c1 cos βx + c2 sin βx) (C)</p> Signup and view all the answers

Which statement describes the method for obtaining a particular integral?

<p>Can be used generally for any given case (A)</p> Signup and view all the answers

What is represented by the expression $A_1 e^{m_1 x} + A_2 e^{m_2 x} + ... + A_n e^{m_n x}$ in the context provided?

<p>The complementary function (B)</p> Signup and view all the answers

Which form is used when there are three real and equal roots in the solution?

<p>(c1 + c2 x + c3 x^2) em1 x + c4 em4 x + ... (B)</p> Signup and view all the answers

What is the significance of the symbol D in the context of differential equations?

<p>It represents a differential operator (C)</p> Signup and view all the answers

How is the complementary function for the case of a pair of equal imaginary roots expressed?

<p>eax[(c1 + c2x) cos βx + (c3 + c4x) sin βx] (C)</p> Signup and view all the answers

Which differential operator corresponds to the equation (D3 + 2D2 + D)y = x2e2x + sin2 x?

<p>Third Order (D)</p> Signup and view all the answers

What is the general form of the differential equation represented by (D2 + 4D + 3)y = e^x sin x + xe^3x?

<p>Second Order Non-Homogeneous (C)</p> Signup and view all the answers

In the equation (D2 + 1)^2 y = x^4 + 2 sin x cos 3x, what type of solution approaches should be considered to solve it?

<p>Method of Variation of Parameters (B)</p> Signup and view all the answers

For the equation (D2 + 2D + 1)y = x cos x, which characteristic of the solution is indicated?

<p>Repeated Roots in Characteristic Equation (A)</p> Signup and view all the answers

How does the equation 2(dy/dx) + 3y = e^x cos x relate to linear differential equations?

<p>It is a linear first-order differential equation. (B)</p> Signup and view all the answers

Which operator accurately reflects the structure of the equation (D2 + 6D + 11)y = 6 + cos 2x?

<p>Second Order Non-Homogeneous (D)</p> Signup and view all the answers

Which method would be suitable for solving (D3 + 5D2 + 7D)y = e^2x cosh x?

<p>Exact Solution via Laplace Transforms (C)</p> Signup and view all the answers

What does the equation (D^2 + 1)^2 y = x^4 + 2 sin x cos 3x indicate about its general solution?

<p>It will have complex conjugate roots contributing oscillatory components. (D)</p> Signup and view all the answers

When can the equation $sin(ax + b) = x$ be applied?

<p>When $f'(a^2) eq 0$ (C)</p> Signup and view all the answers

Under which condition does $cos(ax + b) = x imes cos(ax + b)$ hold true?

<p>When $f'(a^2) = 0$ (A), When $f(a^2) eq 0$ (B)</p> Signup and view all the answers

What is required for the equation $cos(ax + b) = cos(ax + b)$ to be used?

<p>If $f'(a^2) eq 0$ (A)</p> Signup and view all the answers

What conditions apply when $f'(a^2) = 0$?

<p>The function must switch to considering higher order derivatives (B)</p> Signup and view all the answers

What is an example of a situation where $sin(ax + b) = x^2 imes sin(ax + b)$ is true?

<p>When $f'(a^2) = 0$ and $f''(a^2) eq 0$ (C)</p> Signup and view all the answers

Which equation reflects the initial condition of the differential operator $D$ when addressing $cos(2x)$?

<p>$(D^3 + 1)y = cos(2x)$ (D)</p> Signup and view all the answers

What must be satisfied for $sin(ax + b)$ to be equated with $x$ in a function?

<p>f'(a^2) must not equal zero (D)</p> Signup and view all the answers

In the function $f(D) = 2$, what result would be expected if $f'(D)$ is non-zero?

<p>The function behaves normally (D)</p> Signup and view all the answers

What is the complementary function (C.F.) of the differential equation given?

<p>(c1 + c2x) e2x (A)</p> Signup and view all the answers

Which expression correctly represents the particular integral (P.I.) derived from the equation?

<p>8e2x (x2 sin 2x) (B)</p> Signup and view all the answers

What method is used to find the roots of the characteristic equation D^2 - 4D + 4 = 0?

<p>Factoring (A)</p> Signup and view all the answers

What is the general solution of the differential equation after combining the C.F. and P.I.?

<p>y = (c1 + c2x) e2x + 8e2x (x2 sin 2x) (C)</p> Signup and view all the answers

How is the particular integral evaluated in terms of integration?

<p>Using the integral involving sin 2x (C)</p> Signup and view all the answers

What form does the complete solution take when expressed as a sum of functions?

<p>C.F. + P.I. (B)</p> Signup and view all the answers

What does the term 'C.S.' refer to in the context of solving the differential equation?

<p>Complete Solution (B)</p> Signup and view all the answers

In the given context, how many constants of integration are involved in the C.F.?

<p>Two constants (A)</p> Signup and view all the answers

What is the complementary function (C.F.) of the equation $D^2 - 4y = 0$?

<p>c1e^2x + c2e^{-2x} (B)</p> Signup and view all the answers

What is the particular integral (P.I.) for the equation $y'' - 4y = x \sinh x$?

<p>$\frac{x}{2} (e^x - e^{-x})$ (B)</p> Signup and view all the answers

In the solution process for $D^2 - 4y = x \sinh x$, what form does the auxiliary equation take?

<p>$D^2 - 4 = 0$ (A)</p> Signup and view all the answers

Which of the following represents the homogeneous solution for the differential equation $D^2y - 4y = 0$?

<p>c1e^{2x} + c2e^{-2x} (C)</p> Signup and view all the answers

What additional term is included in the complementary solution when considering the full equation involving $sinh x$?

<p>1/2 x sinh x (A)</p> Signup and view all the answers

The expression $\frac{e^x - e^{-x}}{2}$ corresponds to which of the following?

<p>sinh x (D)</p> Signup and view all the answers

What can be concluded about the term $3D^2 + 2D + 2$ when applied to an equation?

<p>It is a second-order polynomial with constant coefficients. (D)</p> Signup and view all the answers

What function represents the general solution $y = c_1 e^x + c_2 e^{2x} + P.I.$ for the equation $D^2 - 4y = 0$?

<p>y = c_1 e^x + c_2 e^{2x} + \frac{x}{2} (e^x - e^{-x})$ (B)</p> Signup and view all the answers

In solving for $D^2 - 4y = x sinh x$, what is the significance of calculating the roots of the characteristic equation?

<p>It determines the structure of the complementary function. (B)</p> Signup and view all the answers

Flashcards

Slope of Tangent

At any point on a curve, the slope of the tangent line is given by the derivative dy/dx.

Equation of Tangent

The equation of the tangent line at point P(x, y) is given by y = (dy/dx)(X - x) + y, where X and Y are the coordinates on the tangent line.

Length of Sub-Tangent

The length of the sub-tangent is the distance along the x-axis between the point of contact and the point where the tangent intersects the x-axis. It is calculated as y(dx/dy).

Length of Sub-Normal

The length of the sub-normal is the distance along the y-axis between the point of contact and the point where the normal intersects the y-axis. It is calculated as y(dy/dx).

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Polar Sub-Tangent

The polar sub-tangent is the distance along the x-axis between the point of contact and the point where the tangent intersects the x-axis in polar coordinates. It is calculated as r^2(dθ/dr).

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Polar Sub-Normal

The polar sub-normal is the distance along the y-axis between the point of contact and the point where the normal intersects the y-axis in polar coordinates. It is calculated as dr/dθ.

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Orthogonal Trajectories

Two families of curves are orthogonal trajectories if every member of one family intersects every member of the other family at a right angle.

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Finding Orthogonal Trajectories (Cartesian)

  1. Form the differential equation of the given family of curves. 2. Replace dy/dx with dx/dy in the differential equation. 3. Solve the resulting differential equation to find the orthogonal trajectories.
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Finding Orthogonal Trajectories (Polar)

  1. Form the differential equation of the given family of curves in polar coordinates. 2. Replace dr/dθ with -r^2(dθ/dr) in the differential equation. 3. Solve the resulting differential equation to find the orthogonal trajectories.
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Newton's Second Law

The net force acting on an object is equal to the product of its mass and acceleration.

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Hooke's Law

The tension in an elastic spring is proportional to the extension or compression of the spring beyond its natural length.

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Resisted Motion Equation (Drag force proportional to velocity)

The equation of motion for a particle moving under gravity with a drag force proportional to velocity is given by mv' = mg - kv, where m is mass, v is velocity, g is gravity, and k is the drag coefficient.

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Resisted Motion Equation (Drag force proportional to velocity squared)

The equation of motion for a particle moving under gravity with a drag force proportional to the square of velocity is given by mv' = mg - kv^2, where m is mass, v is velocity, g is gravity, and k is the drag coefficient.

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Terminal Velocity

The constant velocity that a freely falling object eventually reaches when the force of gravity is balanced by the drag force.

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Escape Velocity

The minimum velocity that an object needs to escape the gravitational pull of a planet or star.

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Centrifugal Force

An outward force that acts on a body moving in a circular path, directed away from the center of the circle. It is caused by the body's inertia.

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Equation of a Rotating Liquid Surface

The shape of the free surface of a liquid rotating in a cylindrical container is described by the equation y = (ω^2/2g)x^2 - (ω^2/2g)r^2/2 + h, where ω is the angular velocity, g is gravity, r is the radius of the cylinder, and h is the initial depth of the liquid.

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Discharge Rate of Water Through a Hole

The rate at which water flows out of a small hole in a container is proportional to the square root of the height of the water level above the hole, expressed as Q = k√h, where Q is the discharge rate, k is a constant, and h is the height of the water level.

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Atmospheric Pressure Equation (Constant Temperature)

The atmospheric pressure at a height z above sea level is given by P = P0e^(gz/k), where P0 is the pressure at sea level, g is gravity, and k is a constant related to the temperature.

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Kirchhoff's Current Law (KCL)

The algebraic sum of currents entering a node is equal to the algebraic sum of currents leaving the node.

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Kirchhoff's Voltage Law (KVL)

The algebraic sum of the voltage drops around a closed loop is equal to the algebraic sum of the electromotive forces (EMFs) in the loop.

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Voltage Drop Across Resistance

The voltage drop across a resistor is proportional to the current flowing through it, given by V_R = IR, where V_R is the voltage, I is the current, and R is the resistance.

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Voltage Drop Across Inductance

The voltage drop across an inductor is proportional to the rate of change of current flowing through it, given by V_L = L(di/dt), where V_L is the voltage, L is the inductance, and i is the current.

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Voltage Drop Across Capacitance

The voltage drop across a capacitor is proportional to the charge stored on it, given by V_C = Q/C, where V_C is the voltage, Q is the charge, and C is the capacitance.

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RL Circuit Equation (Constant EMF)

The differential equation for a series circuit containing a resistor (R) and an inductor (L) driven by a constant voltage source (E) is given by L(di/dt) + Ri = E, where i is the current.

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RL Circuit Equation (Sinusoidal EMF)

The differential equation for a series circuit containing a resistor (R) and an inductor (L) driven by a sinusoidal voltage source (E sin(ωt)) is given by L(di/dt) + Ri = E sin(ωt), where i is the current.

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RLC Circuit Equation (Constant EMF)

The differential equation for a series circuit containing a resistor (R), an inductor (L), and a capacitor (C) driven by a constant voltage source (E) is given by L(d^2q/dt^2) + R(dq/dt) + q/C = E, where q is the charge on the capacitor.

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Solution of the First Order Differential Equation

The solution to a first-order differential equation is a function that satisfies the equation. This solution can be found using various methods, including separation of variables, integrating factors, and the Bernoulli method.

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Initial Conditions

The initial conditions of a differential equation specify the value of the dependent variable and its derivatives at a particular time or point. They are needed to obtain a unique solution to the differential equation.

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Linear Differential Equation

An equation involving a dependent variable and its derivatives, where they appear only in the first degree and are not multiplied together.

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Order of a Differential Equation

The highest order derivative present in the equation determines its order.

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Complementary Function (CF)

The solution to the homogeneous linear differential equation (right-hand side is zero).

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Particular Integral (PI)

A specific solution that satisfies the non-homogeneous differential equation (right-hand side is non-zero).

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General Solution

The sum of the complementary function (CF) and the particular integral (PI).

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Linear Dependence of Solutions

Solutions are linearly dependent if one can be expressed as a linear combination of the others.

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Operator D

Represents differentiation with respect to the independent variable (usually x).

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Inverse Operator D^-1

Represents integration, the inverse operation of differentiation.

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Complementary Function

The solution to the homogeneous differential equation (right-hand side = 0) that represents the natural behavior of the system.

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Particular Integral

A specific solution to the non-homogeneous differential equation that satisfies the forcing function on the right-hand side.

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Auxiliary Equation (A.E.)

The characteristic equation obtained by replacing the differential operator 'D' with a variable (usually 'm') in the homogeneous differential equation.

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Roots of A.E.

The solutions to the Auxiliary Equation, which determine the form of the complementary function.

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C.F. (Complementary Function)

The general solution of the homogeneous differential equation, expressed in terms of exponential functions.

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Working Procedure

A step-by-step method to find the general solution of a non-homogeneous linear differential equation.

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Symbolic Form

A concise way to represent a differential equation using the differential operator 'D' and its powers.

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Solving the A.E. for 'D'

Finding the values of 'D' that satisfy the auxiliary equation, which are the roots of the equation.

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Symbolic Form of DE

Representing a differential equation using the operator 'D' to denote differentiation with respect to the independent variable. For example, d²y/dx² - 4y = x sinh x becomes (D² - 4)y = x sinh x.

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Why is P.I. Needed?

The particular integral is needed to find the complete solution of a non-homogeneous differential equation because the complementary function alone doesn't account for the specific effects of the forcing function on the right-hand side of the equation.

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General Solution = CF + PI

The general solution to a non-homogeneous differential equation is found by adding the complementary function (CF) and the particular integral (PI). This gives the most general solution considering both the inherent behavior of the system and the effects of the forcing function.

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How to find a PI

You can use various methods to find the particular integral (PI), including:

  • Method of undetermined coefficients
  • Method of variation of parameters
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Operator 'D' Inverse

The inverse operator D⁻¹ represents integration, the opposite of differentiation. It's used to solve for the particular integral when the forcing function involves functions like exponentials or trigonometric functions.

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Linearly Independent Solutions

Solutions to a differential equation are linearly independent if none of them can be expressed as a linear combination of the others. This means they are fundamentally different and contribute unique behavior to the overall solution.

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How to find Linearly Independent Solutions

To find linearly independent solutions, you can use the Wronskian determinant. If the determinant of the Wronskian is non-zero, then the solutions are linearly independent.

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P.I. of a differential equation

The Particular Integral (P.I.) is a specific solution to a non-homogeneous differential equation. It represents the part of the solution that is directly affected by the forcing function.

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How to find P.I. for cos(ax + b)

For a differential equation involving a term like cos(ax + b), the P.I. can be found by applying the operator 1/f(D^2) to cos(ax + b) where f(D^2) is the differential operator in the equation.

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Conditions for P.I. of cos(ax + b)

When finding the P.I. of a cos(ax + b) term, conditions apply based on the operator f(D^2). If f(a^2) = 0, you need to apply the operator 1/f'(D^2), and if f'(a^2) = 0, you need to apply 1/f''(D^2) and so on.

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Example: Finding P.I. of (D^3 + 1)y = cos(2x)

To find the P.I. of the equation (D^3 + 1)y = cos(2x), you need to apply the operator 1/f(D^2) = 1/(D^3 + 1) to cos(2x). Since f(-2^2) = -7 ≠ 0, the P.I. is given by 1/(D^3 + 1)cos(2x).

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What is f(D^2) in a differential equation?

In a differential equation, f(D^2) represents the differential operator. It is a polynomial in D^2, where D is the derivative operator (d/dx).

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Why use the operator 1/f(D^2) for P.I.?

Using the operator 1/f(D^2) is a method to find the P.I. of a differential equation involving trigonometric functions. It essentially inverts the effect of the differential operator f(D^2), allowing you to find a solution that satisfies the original equation.

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What if f(a^2) = 0 when finding P.I. of cos(ax + b)?

If f(a^2) = 0, you need to use a different approach to find the P.I. of cos(ax + b). You should replace the operator 1/f(D^2) with 1/f'(D^2) and apply it to the trigonometric term instead.

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What happens if f'(a^2) = 0 when finding the P.I. of cos(ax + b)?

If f'(a^2) = 0, you need to use another alternative. Replace the operator 1/f(D^2) with 1/f''(D^2) and apply it to the cos(ax + b) term for the P.I.

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Differential Equation (DE)

An equation that relates a function with its derivatives.

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Order of a DE

The highest order derivative in the equation.

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Linear DE

An equation where the dependent variable and its derivatives appear only to the first power and are not multiplied together.

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Homogeneous DE

A linear DE where the right-hand side is zero.

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Non-Homogeneous DE

A linear DE where the right-hand side is not zero.

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What is the purpose of the complementary function (CF) in solving a non-homogeneous differential equation?

The complementary function provides the general solution to the homogeneous part of the differential equation, acting as the 'base' solution for the overall problem.

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What is the purpose of the particular integral (PI) in solving a non-homogeneous differential equation?

The particular integral represents a specific solution that satisfies the non-homogeneous part of the equation, accounting for the influence of the specific external forcing function.

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How do you obtain the general solution of a non-homogeneous differential equation?

You combine the complementary function (CF), which represents the intrinsic behavior, with the particular integral (PI), which captures the response to the specific forcing function.

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Study Notes

Applications of Differential Equations of First Order

  • Differential equations of the first order are used to model practical problems.
  • Geometric applications involve curves, tangents, normals, subtangents, and subnormals.
  • Cartesian coordinates are used to find the slope and equations of tangents and normals.
  • Polar coordinates enable calculation of polar subtangents and subnormals.
  • Orthogonal trajectories are curves that intersect every member of a given family of curves at right angles.
  • Physical applications, such as simple electric circuits, Newton's law of cooling, heat flow, radioactive decay, chemical reactions, resisted motion, vertical motion, and orbital motion problems are solved using first order differential equations.

Geometric Applications

  • Cartesian coordinates:

    • Slope of tangent at a point (x, y) on a curve f(x, y) = 0 is dy/dx.
    • Equation of tangent at (x, y) is Y - y1 = (X - x1)(dy/dx).
    • X-intercept (tangent) is x - y(dx/dy).
    • Y-intercept (tangent) is y - x(dy/dx).
    • Equation of normal at (x, y) is Y - y1 = - (X -x1)(dx/dy).
    • Length of tangent is y√(1 + (dx/dy)²).
    • Length of normal is y√(1 + (dy/dx)²).
    • Length of subtangent is y(dx/dy).
    • Length of subnormal is y(dy/dx).
    • Radius of curvature at (x, y) is p = [(1 + (dy/dx)²)³/²]/|d²y/dx²|.
    • Differential of area is ydx or xdy.
  • Polar coordinates:

    • ψ = θ + φ.
    • tan φ = r(dθ/dr).
    • p = r sin φ.
    • p² = r² + (dr/dθ)²

Orthogonal Trajectories

  • Two families of curves are orthogonal trajectories if each member of one family cuts each member of the other family at right angles.
  • To find orthogonal trajectories of a family of curves F(x, y, c) = 0:
    • Eliminate the constant c from the given equation to form its differential equation f(x, y, dy/dx) = 0.
    • Replace (dy/dx) with - (dx/dy) in the differential equation.
    • Solve the resulting differential equation to obtain the orthogonal trajectories.

Physical Applications

  • Simple electric circuits:

    • The voltage drops across the components (resistance, inductance, and capacitance) in a circuit are related to the current and its rate of change.
    • Kirchhoff's laws (voltage and current) can be used to derive differential equations describing the circuit.
  • Newton's law of cooling:

The rate of change of temperature of an object is proportional to the difference between its temperature and the surrounding temperature.

  • Heat flow:

The rate of heat flow through a material is proportional to the temperature gradient.

  • Radioactive decay:

The rate of decay of a radioactive material is proportional to the amount of material remaining.

  • Chemical reactions:

The rate of a chemical reaction may depend on the concentrations of the reactants.

  • Resisted motion:

  • A moving body is opposed by a force per unit mass, proportional to the displacement x and the velocity squared b v².

  • Vertical motion:

    • The forces acting on a falling particle are its weight mg and resistance (proportional to velocity squared) mλv upwards,
  • Orbital Motion:

  • The acceleration of a falling particle is proportional to 1/r^2 where r is the distance from the center.

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Explore the practical applications of first order differential equations, covering both geometric and physical contexts. This quiz delves into Cartesian and polar coordinates, tangent and normal lines, and real-world scenarios like cooling and decay. Test your understanding of these concepts through problem-solving and application-based questions.

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