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Questions and Answers

Which of the following is the correct compound angle identity for $\cos(\alpha + \beta)$?

  • $\cos \alpha \cos \beta + \sin \alpha \sin \beta$
  • $\sin \alpha \cos \beta - \cos \alpha \sin \beta$
  • $\cos \alpha \cos \beta - \sin \alpha \sin \beta$ (correct)
  • $\sin \alpha \cos \beta + \cos \alpha \sin \beta$

The derivation of the compound angle formula for $\cos(\alpha - \beta)$ utilizes which of the following mathematical tools?

  • Pythagorean theorem only
  • Distance formula and the cosine rule (correct)
  • Sine rule only
  • Tangent function and sine rule

Given that $\cos(-\beta) = \cos \beta$ and $\sin(-\beta) = -\sin \beta$, which compound angle formula is directly derived using these identities?

  • $\sin(\alpha - \beta)$
  • $\cos(\alpha + \beta)$ (correct)
  • $\sin(\alpha + \beta)$
  • $\cos(\alpha - \beta)$

To derive the formula for $\sin(\alpha - \beta)$, which co-function identity is used in conjunction with the cosine compound angle formula?

<p>$\sin(x) = \cos(90^\circ - x)$ (C)</p> Signup and view all the answers

Which of the following is a correct double angle formula for $\cos(2\alpha)$?

<p>$\cos^2 \alpha - \sin^2 \alpha$ (D)</p> Signup and view all the answers

Using the sum formula for sine, $\sin(\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$, how is the double angle formula for $\sin(2\alpha)$ derived?

<p>Setting $\beta = \alpha$ (A)</p> Signup and view all the answers

Given the Pythagorean identity $\sin^2 \alpha + \cos^2 \alpha = 1$, which of the following is a valid alternative form of the double angle formula $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$?

<p>$\cos(2\alpha) = 1 - 2\sin^2 \alpha$ (A)</p> Signup and view all the answers

Which of the following represents the double angle formula for $\sin(2\alpha)$?

<p>$2 \sin \alpha \cos \alpha$ (D)</p> Signup and view all the answers

When solving trigonometric equations, what is the purpose of using the CAST diagram?

<p>To determine where trigonometric functions are positive or negative. (B)</p> Signup and view all the answers

What is the general solution for $\tan \theta = x$, where $k$ is an integer?

<p>$\theta = \tan^{-1} x + k \cdot 180^\circ$ (C)</p> Signup and view all the answers

When solving $\sin \theta = x$, why are there generally two sets of solutions, $\theta = \sin^{-1} x + k \cdot 360^\circ$ and $\theta = 180^\circ - \sin^{-1} x + k \cdot 360^\circ$?

<p>Because sine is positive in two quadrants. (B)</p> Signup and view all the answers

In the context of solving trigonometric equations, what does 'k' represent in the general solutions?

<p>An integer representing any whole number of periods. (B)</p> Signup and view all the answers

Which rule is most appropriate for finding the area of a triangle when no perpendicular height is given?

<p>Area Rule (B)</p> Signup and view all the answers

Under what conditions is the Cosine Rule most suitable for solving a triangle?

<p>When two sides and the included angle are known. (B)</p> Signup and view all the answers

When should the Sine Rule be applied to solve a triangle?

<p>When two sides and a non-included angle are given. (A)</p> Signup and view all the answers

Given a triangle ABC, where $a$, $b$, and $c$ are the sides opposite angles $A$, $B$, and $C$ respectively, which equation correctly represents the Cosine Rule for finding side $a$?

<p>$a^2 = b^2 + c^2 - 2bc \cos A$ (B)</p> Signup and view all the answers

To find the height ($h$) of a pole $TF$ with base $F$, observed from points $A$ and $B$ at a distance $d$ apart, which of the following correctly applies the Sine Rule in $\triangle FAB$ and the Tangent Ratio in $\triangle TFB$?

<p>$h = \frac{d \sin \alpha}{\sin \beta} \tan \beta$ (D)</p> Signup and view all the answers

Given $BC = b$, $\angle DBA = \alpha$, $\angle DBC = \beta$, and $\angle DCB = \theta$, how would you correctly express the height $h$ of building $AD$?

<p>$h = \frac{b \sin \alpha \sin \theta}{\sin(\beta + \theta)}$ (A)</p> Signup and view all the answers

A surveyor needs to determine the height of a remote mountain. From a point A, the angle of elevation to the peak is $\alpha$. The surveyor then moves a distance $d$ to point B, where the angle of elevation is $\beta$. Assuming the points A, B, and the base of the mountain are collinear, and given that $\alpha < \beta$, what additional information would be MOST crucial to accurately calculate the mountain's height using trigonometric principles?

<p>The angle formed between the line connecting A and B and the line from A to the base of the mountain if it's not a straight line. (B)</p> Signup and view all the answers

Consider two observers, Alice and Bob, attempting to measure the height of a cloud directly overhead. Alice is at location A, and Bob is at location B, a known distance $d$ apart. Alice measures the angle of elevation to the cloud as $\alpha$, but due to faulty equipment, Bob's angle of elevation measurement, $\beta$, is unreliable and potentially significantly skewed. Instead, Bob accurately measures the angle $\gamma$ formed at point B between the line of sight to the cloud and the line AB connecting their positions. Given that $\alpha$, $\gamma$, and $d$ are known with high precision, which trigonometric approach would yield the MOST accurate determination of the cloud's height, minimizing the impact of Bob's potentially flawed elevation angle ($\beta$) measurement given only $\alpha$, $\gamma$, and $d$?

<p>Use the tangent function with Alice's data $(\tan(\alpha) = h/x)$ and solve for $h$ assuming level ground, disregarding Bob's measurements. (A)</p> Signup and view all the answers

How does deriving general solutions for trigonometric equations account for the periodic nature of trigonometric functions?

<p>By adding integer multiples of the function's period to a base solution. (B)</p> Signup and view all the answers

Which of the following expressions is equivalent to $\cos(\alpha - \beta)$?

<p>$\cos \alpha \cos \beta + \sin \alpha \sin \beta$ (D)</p> Signup and view all the answers

To derive the compound angle formula for $\cos(\alpha + \beta)$ from $\cos(\alpha - \beta)$, which trigonometric identity is primarily used?

<p>$\cos(-\beta) = \cos \beta$ (B)</p> Signup and view all the answers

Which identity is used to transform $\cos(90^\circ - \alpha)$ into $\sin \alpha$ when deriving the compound angle formula for $\sin(\alpha - \beta)$?

<p>Co-function identity (D)</p> Signup and view all the answers

Starting from the compound angle formula for $\cos(\alpha + \beta)$, and knowing that $\cos(2\alpha) = \cos(\alpha + \alpha)$, which of the following is the initial expression for $\cos(2\alpha)$ before simplification?

<p>$\cos \alpha \cos \alpha - \sin \alpha \sin \alpha$ (C)</p> Signup and view all the answers

Given $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$ and the Pythagorean identity $\sin^2 \alpha + \cos^2 \alpha = 1$, which of the following is a derived form of $\cos(2\alpha)$ that eliminates $\sin^2 \alpha$?

<p>$2\cos^2 \alpha - 1$ (B)</p> Signup and view all the answers

Which of the following is the double angle formula for $\sin(2\alpha)$?

<p>$2 \sin \alpha \cos \alpha$ (D)</p> Signup and view all the answers

If $\sin \theta = 0.5$, what are the two principal solutions for $\theta$ in the range $[0^\circ, 360^\circ)$?

<p>$30^\circ, 150^\circ$ (D)</p> Signup and view all the answers

For the equation $\cos \theta = x$, the general solutions are given by $\theta = \cos^{-1} x + k \cdot 360^\circ$ and another set of solutions. What is the other set of general solutions?

<p>$\theta = 360^\circ - \cos^{-1} x + k \cdot 360^\circ$ (C)</p> Signup and view all the answers

What is the period of the tangent function, which influences the general solution for $\tan \theta = x$?

<p>$180^\circ$ (A)</p> Signup and view all the answers

When solving trigonometric equations, the CAST diagram is used to determine:

<p>The quadrant(s) where a trigonometric function is positive or negative. (D)</p> Signup and view all the answers

Which rule is most appropriate to calculate the area of a triangle when you are given two sides and the included angle?

<p>Area Rule (B)</p> Signup and view all the answers

In triangle ABC, if you are given all three sides (a, b, c), which rule should you use to find the angles?

<p>Cosine Rule (C)</p> Signup and view all the answers

When is the Sine Rule applicable for solving a triangle?

<p>When two angles and one side, or two sides and a non-included angle are given. (C)</p> Signup and view all the answers

Given a triangle ABC, and sides a, b, c opposite to angles A, B, C respectively, which formula represents the Cosine Rule to find angle A?

<p>$a^2 = b^2 + c^2 - 2bc \cos A$ (D)</p> Signup and view all the answers

In a 3D problem to find the height of a pole, if you have the distance between two observation points and angles of elevation, which trigonometric rule is initially applied in the horizontal triangle formed by the observation points and the base of the pole?

<p>Sine Rule (B)</p> Signup and view all the answers

To calculate the height $h$ of a building $AD$, given $BC = b$, $\angle DBA = \alpha$, $\angle DBC = \beta$, and $\angle DCB = \theta$, the height is expressed as $h = \frac{b \sin \alpha \sin \theta}{\sin(\beta + \theta)}$. Which trigonometric rule is used in $\triangle BCD$ to find $BD$ in terms of given angles and side $BC$?

<p>Sine Rule (C)</p> Signup and view all the answers

Consider the derivation of $\sin(\alpha - \beta)$. Which sequence of trigonometric transformations is applied?

<p>Co-function identity, then cosine sum formula. (C)</p> Signup and view all the answers

In the derivation of $\cos(\alpha - \beta)$ using the distance formula, the expression $KL^2 = 2 - 2\cos(\alpha - \beta)$ is equated to which of the following based on coordinate geometry?

<p>$(\cos \alpha - \cos \beta)^2 + (\sin \alpha - \sin \beta)^2$ (A)</p> Signup and view all the answers

Given the general solution for $\sin \theta = x$ is $\theta = \sin^{-1} x + k \cdot 360^\circ$ or $\theta = 180^\circ - \sin^{-1} x + k \cdot 360^\circ$, what does the term $k \cdot 360^\circ$ represent in these solutions?

<p>The period of the sine function multiplied by an integer. (C)</p> Signup and view all the answers

Why does the general solution for $\tan \theta = x$ only require adding multiples of $180^\circ$ (i.e., $\theta = \tan^{-1} x + k \cdot 180^\circ$), whereas for $\sin \theta = x$ and $\cos \theta = x$, multiples of $360^\circ$ are added?

<p>Because the period of the tangent function is $180^\circ$, while the period of sine and cosine functions is $360^\circ$. (C)</p> Signup and view all the answers

A surveyor measures the angle of elevation to the top of a mountain from two different points. To accurately calculate the mountain's height using trigonometry, what is the most critical assumption regarding the two points of observation and the mountain?

<p>The observation points and the base of the mountain are collinear in a horizontal plane. (D)</p> Signup and view all the answers

When solving a 3D trigonometry problem involving heights and distances, why is it often beneficial to first solve for a length in a horizontal triangle before calculating the vertical height?

<p>A horizontal length can act as a common side linking the horizontal and vertical triangles, allowing for height calculation using vertical angles. (B)</p> Signup and view all the answers

In the context of deriving trigonometric identities, what general mathematical principle underlies the manipulation and transformation of trigonometric expressions?

<p>Algebraic manipulation and substitution using known identities. (D)</p> Signup and view all the answers

Consider using the double angle formula for $\cos(2\alpha) = 1 - 2\sin^2 \alpha$. In what situation might this form be particularly advantageous compared to $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$ or $\cos(2\alpha) = 2\cos^2 \alpha - 1$?

<p>When simplifying expressions where $\sin^2 \alpha$ terms need to be directly substituted or eliminated. (B)</p> Signup and view all the answers

Which of the following equations correctly represents the compound angle identity for $\sin(\alpha + \beta)$?

<p>$\sin(\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$ (D)</p> Signup and view all the answers

When deriving the compound angle formula for $\cos(\alpha + \beta)$ from $\cos(\alpha - \beta)$, what trigonometric property is utilized?

<p>The negative angle identities (A)</p> Signup and view all the answers

Which of the following co-function identities is used to derive the $\sin(\alpha - \beta)$ formula from a cosine compound angle formula?

<p>$\cos(90^\circ - \alpha) = \sin \alpha$ (D)</p> Signup and view all the answers

Given the compound angle formula for $\cos(\alpha + \beta)$, which of the following is the initial step to derive the double angle formula for $\cos(2\alpha)$?

<p>Replace $\beta$ with $\alpha$ (A)</p> Signup and view all the answers

Starting with $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$ and using the Pythagorean identity, derive an equivalent expression for $\cos(2\alpha)$ that only involves $\cos^2 \alpha$.

<p>$\cos(2\alpha) = 2\cos^2 \alpha - 1$ (B)</p> Signup and view all the answers

What is the simplified form of $\sin(\alpha + \alpha)$ using the sine compound angle formula?

<p>$\sin(2\alpha) = 2 \sin \alpha \cos \alpha$ (A)</p> Signup and view all the answers

Given $\sin \theta = -0.5$, which quadrants would contain the angles that satisfy this condition within the range of $0^\circ$ to $360^\circ$?

<p>Quadrants III and IV (C)</p> Signup and view all the answers

If $\cos \theta = -\frac{\sqrt{3}}{2}$, what are the general solutions for $\theta$?

<p>$\theta = 150^\circ + k \cdot 360^\circ$ or $\theta = 210^\circ + k \cdot 360^\circ$ (C)</p> Signup and view all the answers

For the general solution of $\tan \theta = 1$, why is the period multiplied by $k$ equal to $180^\circ$ rather than $360^\circ$?

<p>Because the tangent function repeats every $180^\circ$ (B)</p> Signup and view all the answers

In solving trigonometric equations, what primary role does the CAST diagram serve?

<p>To determine the signs of trigonometric functions in different quadrants. (D)</p> Signup and view all the answers

When no height is given, which rule is best used to determine a triangle's area?

<p>Area Rule (C)</p> Signup and view all the answers

You are given all three sides of a triangle and need to find the measure of one of the angles. Which rule is most appropriate for this task?

<p>Cosine Rule (D)</p> Signup and view all the answers

Given two sides and a non-included angle of a triangle, which rule should be applied to solve the triangle?

<p>Sine Rule (D)</p> Signup and view all the answers

For triangle $ABC$, which equation correctly represents the Cosine Rule to find angle $A$, given sides $a$, $b$, and $c$?

<p>$\cos A = \frac{b^2 + c^2 - a^2}{2bc}$ (A)</p> Signup and view all the answers

When dealing with 3D problems involving a vertical pole and observations from two points on the ground, which trigonometric relationship is primarily used in the vertical triangle formed by the pole and the distance to the observation point?

<p>Tangent Ratio (D)</p> Signup and view all the answers

In a scenario where you are trying to find the height $h$ of a building $AD$, and you have $BC = b$, $\angle DBA = \alpha$, $\angle DBC = \beta$, and $\angle DCB = \theta$, which trigonometric principle is applied in $\triangle BCD$ to express $BD$ in terms of the given sides and angles?

<p>Sine Rule (D)</p> Signup and view all the answers

Which of the following sequences of transformations correctly describes the derivation of the formula for $\sin(\alpha - \beta)$?

<p>Apply co-function identity, then cosine sum formula. (B)</p> Signup and view all the answers

In the derivation of $\cos(\alpha - \beta)$ using the distance formula in coordinate geometry, the expression $KL^2 = 2 - 2\cos(\alpha - \beta)$ is equated to which of the following?

<p>The square of the distance between points $K$ and $L$ derived from coordinate differences (D)</p> Signup and view all the answers

What does the term $k \cdot 360^\circ$ represent in the general solutions for $\sin \theta = x$ and $\cos \theta = x$?

<p>All possible solutions due to the periodic nature of sine and cosine (A)</p> Signup and view all the answers

A surveyor is tasked with determining the height of a tall building. They measure the angle of elevation from two points on the ground. What essential assumption must they make to accurately apply trigonometric principles in this scenario?

<p>The two points and the base of the building are collinear. (C)</p> Signup and view all the answers

While trying to determine the height of a cliff using angles of elevation from two land-based observation points, a surveyor finds that one measurement was imprecise. He only has one accurate angle of elevation ($\alpha$), the direct distance ($d$) between the points $A$ and $B$, and the angle ($\gamma$) formed at point B between the line of sight of the cliff and the line $AB$. How can he find the height?

<p>Solve $\triangle ABC$ using the given data and the sine rule. (C)</p> Signup and view all the answers

What is the primary reason that solving for a length in a horizontal triangle is strategically advantageous before calculating vertical height in many 3D trigonometry problems?

<p>It provides a baseline for applying trigonometric ratios to find the height. (D)</p> Signup and view all the answers

Suppose you need to solve a complex trigonometric equation involving $\cos(2\alpha)$, and you know the value of $\sin \alpha$. Which form of the double angle formula for $\cos(2\alpha)$ would be most efficient to use?

<p>$\cos(2\alpha) = 1 - 2 \sin^2 \alpha$ (B)</p> Signup and view all the answers

You are tasked with determining the angle at which a beam of light must be directed from point $A$ to illuminate a solar panel at point $B$. The coordinates of $A$ are $(x_1, y_1, z_1)$, and the coordinates of $B$ are $(x_2, y_2, z_2)$ in a three-dimensional space. There are obstructions that make direct measurement impossible, and the angles must be calculated through trigonometry with the aid of intermediate reference points. Identify the MOST critical factor to ensure an accurate calculation of the required angles.

<p>Establishing a consistent coordinate system and accurately determining the spatial relationships between $A$, $B$, and any reference points. (B)</p> Signup and view all the answers

Flashcards

$\cos(\alpha - \beta)$ Formula

States that $\cos(\alpha - \beta) = \cos \alpha \cos \beta + \sin \alpha \sin \beta$.

$\cos(\alpha + \beta)$ Formula

States that $\cos(\alpha + \beta) = \cos \alpha \cos \beta - \sin \alpha \sin \beta$.

$\sin(\alpha - \beta)$ Formula

States that $\sin(\alpha - \beta) = \sin \alpha \cos \beta - \cos \alpha \sin \beta$.

$\sin(\alpha + \beta)$ Formula

States that $\sin(\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$.

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$\sin(2\alpha)$ Formula

$\sin(2\alpha) = 2 \sin \alpha \cos \alpha$. Used to express the sine of double an angle.

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$\cos(2\alpha)$ Formula (Form 1)

$\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$, expresses cosine of double angle.

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$\cos(2\alpha)$ Formula (Form 2)

An alternate form of the double angle formula: $\cos(2\alpha) = 2\cos^2 \alpha - 1$.

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$\cos(2\alpha)$ Formula (Form 3)

An alternate form of the double angle formula: $\cos(2\alpha) = 1 - 2\sin^2 \alpha$.

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General Solution

A method used to find all possible solutions to a trigonometric equation due to their periodic nature.

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General Solution Method

First, simplify the equation. Then find the reference angle, use the CAST diagram, find restricted values and obtain the general solution.

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General Solution for $\sin \theta = x$

$\theta = \sin^{-1} x + k \cdot 360^\circ$ or $\theta = 180^\circ - \sin^{-1} x + k \cdot 360^\circ$, where $k$ is an integer.

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General Solution for $\cos \theta = x$

$\theta = \cos^{-1} x + k \cdot 360^\circ$ or $\theta = 360^\circ - \cos^{-1} x + k \cdot 360^\circ$, where $k$ is an integer.

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General Solution for $\tan \theta = x$

$\theta = \tan^{-1} x + k \cdot 180^\circ$, where $k$ is an integer.

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Area Rule

Area $= \frac{1}{2}bc \sin A = \frac{1}{2}ac \sin B = \frac{1}{2}ab \sin C$. Used when no perpendicular height is given.

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Sine Rule

$\frac{\sin A}{a} = \frac{\sin B}{b} = \frac{\sin C}{c}$. Used with two angles and a side or two sides and a non-included angle.

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Cosine Rule

$a^2 = b^2 + c^2 - 2bc \cos A$; relates sides and angles in a triangle.

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General Approach to 3D Problems

First, draw a sketch, consider the given information link sides/angles, apply appropriate rules, then calculate the desired quantities.

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Height of a Pole Formula

When (AB = d), (\angle FBA = \theta), (\angle FAB = \alpha), (\angle FBT = \beta), and (\angle TFB = 90^\circ), the height (h) is given by (h = \frac{d \sin \alpha}{\sin \beta} \tan \beta).

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Height of a Building Formula

When (BC = b), (\angle DBA = \alpha), (\angle DBC = \beta), and (\angle DCB = \theta), the height (h) is given by (h = \frac{b \sin \alpha \sin \theta}{\sin(\beta + \theta)}).

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When to use the Cosine Rule

This rule is used when no right angle is given, and either two sides and the included angle or three sides are given.

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When to use the Sine Rule

This rule is used when no right angle is given, and either two sides and an angle (not the included angle) or two angles and a side are given.

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Cosine Negative Angle Identity

The identity: (\cos(-\beta) = \cos \beta). Cosine is an even function.

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Sine Negative Angle Identity

The identity: (\sin(-\beta) = -\sin \beta). Sine is an odd function.

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Solving Trigonometric Equations

Uses the periodic nature of trigonometric functions to find all possible solutions for a given equation.

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When to use the Area Rule

Used when no perpendicular height is given in the problem.

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Double Angle Formulas Utility

Simplifies trigonometric expressions or equations by expressing functions of multiple angles in terms of single angles.

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Compound Angle Identities

Trigonometric identities that express trigonometric functions of sums or differences of angles in terms of trigonometric functions of the individual angles.

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Study Notes

Compound Angle Identities

  • The cosine of the difference of two angles: $\cos(\alpha - \beta) = \cos \alpha \cos \beta + \sin \alpha \sin \beta$
  • The cosine of the sum of two angles: $\cos(\alpha + \beta) = \cos \alpha \cos \beta - \sin \alpha \sin \beta$
  • The sine of the difference of two angles: $\sin(\alpha - \beta) = \sin \alpha \cos \beta - \cos \alpha \sin \beta$
  • The sine of the sum of two angles: $\sin(\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$

Derivation of $\cos(\alpha - \beta)$

  • Using the distance formula and cosine rule: $KL^2 = (\cos \alpha - \cos \beta)^2 + (\sin \alpha - \sin \beta)^2$
  • Simplifies to: $KL^2 = 2 - 2 \cos(\alpha - \beta)$
  • Equating both expressions gives: $2 - 2 \cos(\alpha - \beta) = 2 - 2 (\cos \alpha \cos \beta + \sin \alpha \sin \beta)$
  • Therefore: $\cos(\alpha - \beta) = \cos \alpha \cos \beta + \sin \alpha \sin \beta$

Derivation of $\cos(\alpha + \beta)$

  • Using the negative angle identity: $\cos(\alpha + \beta) = \cos(\alpha - (-\beta))$
  • Expanding this gives: $\cos \alpha \cos(-\beta) + \sin \alpha \sin(-\beta)$
  • Since $\cos(-\beta) = \cos \beta$ and $\sin(-\beta) = -\sin \beta$: $\cos(\alpha + \beta) = \cos \alpha \cos \beta - \sin \alpha \sin \beta$

Derivation of $\sin(\alpha - \beta)$ and $\sin(\alpha + \beta)$

  • $\sin(\alpha - \beta)$ can be derived using co-functions: $\sin(\alpha - \beta) = \cos(90^\circ - (\alpha - \beta))$
  • Which simplifies to: $ \cos((90^\circ - \alpha) + \beta)$
  • Expanding the cosine sum gives: $\cos(90^\circ - \alpha) \cos \beta - \sin(90^\circ - \alpha) \sin \beta$
  • Since $\cos(90^\circ - \alpha) = \sin \alpha$ and $\sin(90^\circ - \alpha) = \cos \alpha$, it follows that: $\sin(\alpha - \beta) = \sin \alpha \cos \beta - \cos \alpha \sin \beta$
  • Similarly: $\sin(\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$

Summary of Compound Angle Formulas

  • $\cos(\alpha - \beta) = \cos \alpha \cos \beta + \sin \alpha \sin \beta$
  • $\cos(\alpha + \beta) = \cos \alpha \cos \beta - \sin \alpha \sin \beta$
  • $\sin(\alpha - \beta) = \sin \alpha \cos \beta - \cos \alpha \sin \beta$
  • $\sin(\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$

Double Angle Formulas

  • $\sin(2\alpha) = 2 \sin \alpha \cos \alpha$
  • $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$
  • $\cos(2\alpha) = 2\cos^2 \alpha - 1$
  • $\cos(2\alpha) = 1 - 2\sin^2 \alpha$

Sine of Double Angle

  • $\sin(2\alpha) = 2 \sin \alpha \cos \alpha$

Cosine of Double Angle

  • $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$
  • $\cos(2\alpha) = 1 - 2 \sin^2 \alpha$
  • $\cos(2\alpha) = 2 \cos^2 \alpha - 1$

Derivation of $\sin(2\alpha)$

  • Starting with the sum formula for sine: $\sin(\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$
  • Let $\alpha = \beta$, then: $\sin(2\alpha) = \sin(\alpha + \alpha) = \sin \alpha \cos \alpha + \cos \alpha \sin \alpha$
  • Therefore: $\sin(2\alpha) = 2 \sin \alpha \cos \alpha$

Derivation of $\cos(2\alpha)$

  • Starting with the sum formula for cosine: $\cos(\alpha + \beta) = \cos \alpha \cos \beta - \sin \alpha \sin \beta$
  • Let $\alpha = \beta$, then: $\cos(2\alpha) = \cos(\alpha + \alpha) = \cos \alpha \cos \alpha - \sin \alpha \sin \alpha$
  • Thus: $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$

Form 1 for cosine

  • $\cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha$

Form 2 for cosine

  • $\cos(2\alpha) = \cos^2 \alpha - (1 - \cos^2 \alpha)$
  • $\cos(2\alpha) = 2 \cos^2 \alpha - 1$

Form 3 for cosine

  • $\cos(2\alpha) = (1 - \sin^2 \alpha) - \sin^2 \alpha$
  • $\cos(2\alpha) = 1 - 2 \sin^2 \alpha$

Solving Trigonometric Equations

  • Due to periodicity, trigonometric equations have infinite solutions.

General Solution Method

  • Simplify the equation using algebra and trig identities.
  • Determine the reference angle using positive values.
  • Use CAST diagram to identify quadrants where the function is positive or negative.
  • Find angles within a specified interval by adding or subtracting multiples of the period.
  • Express angles in the interval $[0^\circ, 360^\circ]$ which satisfy the equation, and add multiples of the period.
  • Verify the solutions with a calculator.

General Solutions for Common Equations

  • If $\sin \theta = x$: $\theta = \sin^{-1} x + k \cdot 360^\circ$ or $\theta = 180^\circ - \sin^{-1} x + k \cdot 360^\circ$
  • If $\cos \theta = x$: $\theta = \cos^{-1} x + k \cdot 360^\circ$ or $\theta = 360^\circ - \cos^{-1} x + k \cdot 360^\circ$
  • If $\tan \theta = x$: $\theta = \tan^{-1} x + k \cdot 180^\circ$, where $k \in \mathbb{Z}$ (integers).

Area Rule

  • Area of $\triangle ABC$: $\frac{1}{2}bc \sin A = \frac{1}{2}ac \sin B = \frac{1}{2}ab \sin C$

Sine Rule

  • $\frac{\sin A}{a} = \frac{\sin B}{b} = \frac{\sin C}{c}$

Cosine Rule

  • $a^2 = b^2 + c^2 - 2bc \cos A$
  • $b^2 = a^2 + c^2 - 2ac \cos B$
  • $c^2 = a^2 + b^2 - 2ab \cos C$

How to Determine Which Rule to Use:

  • Area Rule: when no perpendicular height is given.
  • Sine Rule: when no right angle is given, and either two sides and an angle (not the included angle) or two angles and a side are given.
  • Cosine Rule: when no right angle is given, and either two sides and the included angle or three sides are given.

Using the Cosine Rule

  • $b^2 = a^2 + c^2 - 2ac \cos B$

Using the Sine Rule

  • $\frac{\sin C}{c} = \frac{\sin B}{b}$

Calculating the Area of a Triangle

  • Area of $\triangle ABC = \frac{1}{2}ac \sin B$

General Approach to 3-Dimensional Problems

  • Draw a Sketch to visualize the problem.
  • Consider the Given Information by identifying relevant triangles and link sides or angles.
  • Apply Appropriate Rules like Sine Rule, Cosine Rule, or trigonometric identities as needed.
  • Calculate the Desired Quantities, such as lengths, angles, or areas.

Height of a Pole

  • Given: $AB = d$, $\angle FBA = \theta$, $\angle FAB = \alpha$, $\angle FBT = \beta$, $\angle TFB = 90^\circ$
  • Using the Sine Rule in $\triangle FAB$: $\frac{FB}{\sin \alpha} = \frac{AB}{\sin(180^\circ - \beta)}$
  • $FB = \frac{d \sin \alpha}{\sin \beta}$
  • Using the Tangent Ratio in $\triangle TFB$: $\tan \beta = \frac{h}{FB}$
  • $h = FB \tan \beta = \frac{d \sin \alpha}{\sin \beta} \tan \beta$

Height of a Building

  • Given: $BC = b$, $\angle DBA = \alpha$, $\angle DBC = \beta$, $\angle DCB = \theta$
  • Using the Sine Rule in $\triangle BCD$: $\frac{BD}{\sin \theta} = \frac{BC}{\sin(\beta + \theta)}$
  • $BD = \frac{b \sin \theta}{\sin(\beta + \theta)}$
  • Using the Sine Rule in $\triangle ABD$: $\sin \alpha = \frac{h}{BD}$
  • $h = BD \sin \alpha = \frac{b \sin \alpha \sin \theta}{\sin(\beta + \theta)}$

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