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Stage 5.1-3

6.05 Applications of trigonometry

Lesson

Introduction

The trigonometric ratios gave us relationships between the sides and angles in a right-angled triangle. In real life situations we won't always know all the information about a right-angled triangle and we can use the trigonometric ratios to find those missing values.

Trigonometry problems

To find missing angles in right-angled triangles using trigonometry, we require at least two known sides and the trigonometric ratios.

Remember that the trigonometric ratios for a right-angled triangle are:

\sin \theta =\dfrac{\text{Opposite }}{\text{Hypotenuse }} \quad \quad \cos \theta =\dfrac{\text{Adjacent }}{\text{Hypotenuse }} \quad \quad \tan \theta =\dfrac{\text{Opposite }}{\text{Adjacent }}

To isolate the angle in each of these relationships, we can apply the inverse trigonometric function to each side of the equation. This will give us:

\theta =\sin^{-1}\left(\dfrac{\text{Opposite }}{\text{Hypotenuse }}\right) \quad \quad \theta =\cos^{-1}\left(\dfrac{\text{Adjacent }}{\text{Hypotenuse }}\right) \quad \quad \theta =\tan^{-1}\left(\dfrac{\text{Opposite }}{\text{Adjacent }}\right)

Any of these relationships can be used to find \theta depending on which side lengths of the triangle are known.

While we may represent the inverse trigonometric functions using an index of -1, they are not reciprocals of the original functions.

For example: \sin ^{-1}\theta \neq \dfrac{1}{\sin \theta }

To find missing sides in right-angled triangles using trigonometry, we require at least one angle (other than the right angle) and at least one other side length.

We can find a missing side length using one of the trigonometric ratios where the missing length the only unknown value. Doing this allows us to solve the equation and find that value.

For example:

A right angle triangle with angle of 26 degrees, adjacent side length of 5 and hypotenuse of x.

In this right-angled triangle, we are given one angle and one side. Using this, we want to find the length x.

Since the given side is adjacent to the given angle and x is the length of the hypotenuse, we can express their relationship using the trigonometric ratio: \cos 26\degree =\frac{5}{x}

We can isolate x in this equation by multiplying both sides by x and then dividing both sides by \cos 26\degree . This will give us: x=\frac{5}{\cos 26\degree }

Solving this tells us that x=5.56, rounded to two decimal places.

Depending on the given angle, given side and missing side, we will need to use one of the three trigonometric ratios so that all relevant information is in the equation.

Examples

Example 1

The person in the picture sights a pigeon above him. If the angle the person is looking at is \alpha, find \alpha in degrees.

A formed right angle triangle with angle a degrees with opposite side of 19 metres and adjacent side of 13 metres.

Round your answer to two decimal places.

Worked Solution
Create a strategy

Use the inverse trigonometric function that relates the two given sides of the right-angled triangle.

Apply the idea

With respect to the given angle of \alpha, the side length of 13\, \text{m} is adjacent and the side length of 19\, \text{m} is opposite, so we can use the inverse tangent ratio.

\displaystyle \alpha\displaystyle =\displaystyle \tan^{-1}\left(\frac{\text{Opposite }}{\text{Adjacent}}\right)Use the inverse tan function
\displaystyle =\displaystyle \tan^{-1}\left(\frac{19}{13}\right)Substitute the values
\displaystyle =\displaystyle 55.62 \degreeEvaluate using calculator and round

Example 2

A helicopter is 344 metres away from its landing pad.

If the angle of depression to the landing pad is 32, what is the height, x, of the helicopter above the ground?

A right angle triangle with a helicopter and a landing pad at different corners. Ask your teacher for more information.

Round your answer to the nearest metre.

Worked Solution
Create a strategy

Use the trigonometric ratio that relates the two sides and substitute the given values.

Apply the idea

With respect to the given angle of 32\degree, the side of length x is opposite and the side of length 344\,\text{m} is the hypotenuse, so we can use the sine ratio.

\displaystyle \sin \theta\displaystyle =\displaystyle \frac{\text{Opposite }}{\text{Hypotenuse}}Use the sine ratio
\displaystyle \sin 32 \degree\displaystyle =\displaystyle \frac{x}{344}Substitute the values and x
\displaystyle x\displaystyle =\displaystyle 344 \sin 32 \degreeMultiply both sides by 344
\displaystyle =\displaystyle 182\ \text{m}Evaluate using calculator

Example 3

A 27.3\, \text{m} long string of lights joins the top of a tree to a point on the ground.

If the tree is 7.4\, \text{m} tall, find \theta, the angle the string of lights would make with the tree to the nearest two decimal places.

Worked Solution
Create a strategy

Sketch the situation to identify the needed trigonometric ratio, then solve for the angle \theta.

Apply the idea

The situation has been sketched below:

A sketched forming a right-angled triangle that joins the top of a tree to the ground. Ask your teacher for more information.

With respect to the given angle of \theta, 7.4 is the adjacent side and 27.3 is the hypotenuse, so we can use the cosine ratio.

\displaystyle \cos \theta\displaystyle =\displaystyle \frac{\text{Adjacent }}{\text{Hypotenuse}}Use the cos ratio
\displaystyle \cos \theta\displaystyle =\displaystyle \frac{7.4}{27.3}Substitute the values
\displaystyle \theta\displaystyle =\displaystyle \cos ^{-1}\left(\frac{7.4}{27.3}\right)Apply the inverse
\displaystyle =\displaystyle 74.27\degreeEvaluate using a calculator
Idea summary

To find missing sides in right-angled triangles using trigonometry, we require at least one angle (other than the right angle) and at least one other side length.

To find missing angles in right-angled triangles using trigonometry, we require at least two known sides.

The trigonometric ratios for a right-angled triangle are:

\sin \theta =\dfrac{\text{Opposite }}{\text{Hypotenuse }} \quad \quad \cos \theta =\dfrac{\text{Adjacent }}{\text{Hypotenuse }} \quad \quad \tan \theta =\dfrac{\text{Opposite }}{\text{Adjacent }}

The inverse trigonometric ratios are:

\theta =\sin^{-1}\left(\dfrac{\text{Opposite }}{\text{Hypotenuse }}\right) \quad \quad \theta =\cos^{-1}\left(\dfrac{\text{Adjacent }}{\text{Hypotenuse }}\right) \quad \quad \theta =\tan^{-1}\left(\dfrac{\text{Opposite }}{\text{Adjacent }}\right)

Outcomes

MA5.1-10MG

applies trigonometry, given diagrams, to solve problems, including problems involving angles of elevation and depression

MA5.2-13MG

applies trigonometry to solve problems, including problems involving bearings

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