Differential Equations and Slope Fields

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Questions and Answers

Which of the following equations represents a third-order differential equation?

  • y'' + 2y' + y = 0
  • $\frac{d^3y}{dt^3} + 3 = 0$ (correct)
  • f'(t) = -1
  • y' = x - y

Newton's derivation of the height of a falling body involves several steps. Which of the following correctly outlines the progression of these steps?

  • Application of initial conditions → initial differential equation → integration to find velocity → integration to find position.
  • Initial differential equation → integration to find velocity → integration to find position → application of initial conditions. (correct)
  • Integration to find position → initial differential equation → integration to find velocity → application of initial conditions.
  • Initial differential equation → integration to find position → integration to find velocity → application of initial conditions.

The rate at which the velocity of an object changes is 4 $m/s^2$. Which differential equation expresses this relationship?

  • $\frac{dv}{dt} = 4$ (correct)
  • $\frac{d^2v}{dt^2} = 4$
  • $\int v(t) dt = 4$
  • $\frac{dv}{dt} = 4t$

The rate at which a drink warms or cools is proportional to the difference between the ambient temperature $T_a$ and the current temperature of the drink $T$. Which equation models this?

<p>$\frac{dT}{dt} = k(T_a - T)$ (D)</p> Signup and view all the answers

If pressure $P$ applied to a gas increases at constant temperature, the rate of change of volume $V$ with respect to pressure is proportional to the reciprocal of the square of the pressure. Which differential equation expresses this?

<p>$\frac{dV}{dP} = \frac{k}{P^2}$ (D)</p> Signup and view all the answers

Determine whether $y = -2x^5$ is a solution to the differential equation $\frac{dy}{dx} = \frac{y}{x}$.

<p>No, $y = -2x^5$ is not a solution because the derivatives do not match. (B)</p> Signup and view all the answers

Given the differential equation $y'' - 2y' - 8y = 0$, which of the following functions is a solution?

<p>$y = Ce^{4x}$ (C)</p> Signup and view all the answers

A slope field visualizes solutions to differential equations. For the differential equation $\frac{dy}{dx} = x + y$, what is the slope at the coordinate point (1, 0)?

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

A slope field for a certain differential equation is given. Which of the following could be the solution to the differential equation with the initial condition y(0) = 0?

<p>$y = \ln(x+1)$ (C)</p> Signup and view all the answers

Given a slope field, which statement about a solution $y = f(x)$ must be false?

<p>The graph of the particular solution that satisfies f(1) = -1 is concave down on the interval (0,2). (B)</p> Signup and view all the answers

Consider the differential equation $\frac{dy}{dx} = \frac{y + 1}{2}$. What steps would you take to find the particular solution passing through the point (1,0)?

<p>Separate variables, integrate, then apply the initial condition. (D)</p> Signup and view all the answers

Consider the given slope field. Which of the following could be a solution to the differential equation with the initial condition $y(0) = 1$?

<p>$y = \frac{1}{1 + x^2}$ (B)</p> Signup and view all the answers

Based on the slope field, which of the following could be a solution, $y = f(x)$, to the differential equation passing through the point (0, 0)?

<p>$y = sin(x)$ (C)</p> Signup and view all the answers

For the differential equation $\frac{dy}{dx} = \frac{x^2}{y}$ for all $y \neq 0$, describe all points in the xy-plane for which $\frac{dy}{dx} = -2$.

<p>y =-x^2/2 ; \text{ for all } y \neq 0 (C)</p> Signup and view all the answers

Given a slope field for a certain differential equation, which of the following could the solution to the differential equation be?

<p>$y = ln(x)$ (D)</p> Signup and view all the answers

Which of the following differential equations is separable?

<p>$\frac{dy}{dx} = \frac{x}{y}$ (A)</p> Signup and view all the answers

What is the general solution to the differential equation $\frac{dy}{dx} = y \sin(x)$?

<p>$y = Ce^{-cos(x)}$ (C)</p> Signup and view all the answers

To find a particular solution to a differential equation, what is the correct order of steps?

<p>Find the general solution, apply the initial condition, solve explicitly for y. (B)</p> Signup and view all the answers

What is the particular solution that satisfies the differential equation $\frac{dy}{dx} = \frac{x+1}{y}$ and the condition $y(0) = 1$?

<p>$y = \sqrt{x^2 + 2x + 2}$ (A)</p> Signup and view all the answers

The number of barrels of oil a company exports annually increases at a rate proportional to the number of barrels exported at that time. Initially, the company exports 5.4 million barrels, and after 6 years, it exports 10.8 million barrels. Which equation expresses number of barrels in terms of time?

<p>$P(t) = 5.4e^{0.693t}$ (D)</p> Signup and view all the answers

A population grows at a rate proportional to its size, represented by $\frac{dP}{dt} = kP$. Which of the following demonstrates the steps to find the general solution?

<p>Separate variables, integrate both sides to find $\ln|P| = kt + C$, then exponentiate to get $P(t) = P_0e^{kt}$ (D)</p> Signup and view all the answers

Consider the differential equation $\frac{dy}{dx} = \frac{x - 2}{y}$. Let $y = f(x)$ be the particular solution satisfying the initial condition $f(1) = 2$. Find an equation of the line tangent to the graph of $y = f(x)$ at $x = 1$.

<p>$y= -x + 3$ (D)</p> Signup and view all the answers

Of the following, which are solutions to the differential equation $y'' + 4y = 0$?

<p>I only (C)</p> Signup and view all the answers

Flashcards

What is a Differential Equation?

An equation that involves an unknown function and its derivatives.

What defines the order of a Differential Equation?

The order corresponds to the highest derivative in the differential equation.

What are solutions to differential equations?

Functions that satisfy the differential equation.

What is a Slope Field?

A visual representation of solutions to a first-order differential equation.

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What is a General Solution?

A solution with an arbitrary constant.

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What is a Particular Solution?

A solution obtained by assigning specific values to the arbitrary constants in the general solution.

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What is Separation of Variables?

A technique to solve differential equations by separating variables.

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What is a Separable Differential Equation?

An equation where terms involving each variable can be isolated on opposite sides of the equation

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What is Acceleration?

The rate at which the velocity of an object is changing.

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Newton's Law of Cooling/Warming

dy/dt = k(Ta - T), where Ta is ambient temperature, T is current temperature, and k is a constant.

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Relationship between Pressure and Volume

The rate of change of the volume of gas with respect to the pressure is proportional to the reciprocal of the square of the pressure.

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What is Exponential Growth?

The model where Population grows at a rate proportional to its size.

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Study Notes

  • Unit 7 focuses on differential equations and their applications.

Learning Objectives

  • Define differential equations.
  • Understand and use slope fields.
  • Find general solutions using separation of variables.
  • Use initial conditions for particular solutions.

Differential Equations Defined

  • A differential equation involves derivatives of a function.
  • Solutions will be a function

Order of Differential Equations

  • The order corresponds to the highest derivative in the equation.
  • y'' + 2y' + 3 = 0 is a second-order differential equation.
  • d³y/dt³ + 3 = 0 is a third-order differential equation.
  • f'(t) = -1 is a first-order differential equation.

Solving Differential Equations

  • Separable differential equations involve isolating variables on each side.
  • Slope fields can visually represent solutions to more complex differential equations.

Newton's Derivation of Falling Body Equation

  • Starts from d²h/dt² = -g, where h is height and g is gravitational acceleration.
  • Integrating dv/dt = -g results in v = -gt + C, where C is a constant.
  • Object's position h(t) integrates to -½gt² + Ct + D, with D as another constant.
  • Constants C and D are determined by initial conditions: initial height (hâ‚€) and initial velocity (vâ‚€).
  • Therefore, h(t) = -½gt² + vâ‚€t + hâ‚€.

Differential Equation Examples

  • "The rate at which the velocity of an object is changing is 4 meters per second per second" can be represented as dv/dt = 4.
  • The rate of drink cooling is proportional to the difference between ambient (Ta) and drink (T) temperatures (dT/dt = k(Ta - T)).
  • Volume change rate is proportional to the inverse square of pressure can be written as dV/dP = k/P².

Verifying Solutions

  • Substitute the proposed solution into the differential equation.
  • Check if it satisfies the equation.

Slope Fields

  • Graphically represent differential equations
  • Tangent lines at (x, y) have a slope of x + y.
  • Curves following tangent lines represent particular solutions
  • These curves are specific solutions from point (2,-1) on a family of general solutions

Separable Differential Equations

  • Can be written in the form g(y) dy = f(x) dx.
  • Example: dy/dx = x/y becomes ∫ydy = ∫xdx.
  • NOT Separable: dy/dx = x - y or dy/dx = e^(xy).

Finding General Solutions

  • Solve the general solution of g'(x) = y sin(x).
  • To find a general solution for dy/dx = (3x²+2) / (5 cos(y)), separation of variables is used.

Solving Explicitly for y

  • Find a general solution to dy/dx = (cos(x)e^(sin(x))) / cos(y) and solve for y.

Solving Differential Equations

  • Formula for all y >0 is dy/dx = y - y sin(x).

Particular Solutions

  • Find the general solution first.
  • Apply initial conditions to find the constant C.
  • Solve for y explicitly if needed.

Example: Finding Particular Solutions

  • Given dy/dx = (x+1) / y
  • Find the general solution.
  • Solve for y(0) = 1 using two methods: unsimplified and simplified.

Exponential Growth/Decay

  • A population grows at a rate proportional to its size is dP/dt = kP (k is proportionality constant).
  • The general solution is P(t) = P(0)e^(kt), where P(0) is the starting population.

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