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# Using the Cosine Function to Find the Adjacent

(KS3, Year 8)

## The Lesson

The cosine function relates a given angle to the adjacent side and hypotenuse of a right triangle. The length of the adjacent is given by the formula below:In this formula,

**θ**is an angle of a right triangle, the adjacent is the length of the side next to the angle and the hypotenuse is the length of longest side. The image below shows what we mean:

## How to Use the Cosine Function to Find the Adjacent of a Right Triangle

Finding the adjacent of a right triangle is easy when we know the angle and the hypotenuse.## Question

What is the length of the adjacent of the right triangle shown below?## Step-by-Step:

# 1

Start with the formula:

Adjacent = cos θ × hypotenuse

# 2

Substitute the angle θ and the length of the hypotenuse into the formula. In our example, θ = 60° and the hypotenuse is 5 cm.

Adjacent = cos (60°) × 5
Adjacent = 0.5 × 5
Adjacent = 2.5

## Answer:

The length of the adjacent of a right triangle with an angle of 60° and a hypotenuse of 5 cm is 2.5 cm.## Remembering the Formula

Often, the hardest part of finding the unknown angle is remembering which formula to use. Whenever you have a right triangle where you know one side and one angle and have to find an unknown side... ......think trigonometry... ...............think sine, cosine or tangent... ........................think**SOH CAH TOA**.

Looking at the example above, we are trying to find the

**A**djacent and we know the

**H**ypotenuse.

The two letters we are looking for are

**AH**, which comes in the

**CAH**in SOH

**CAH**TOA. This reminds us of the equation:

**C**os θ =

**A**djacent /

**H**ypotenuse

**Note**).

**A**djacent =

**C**os θ ×

**H**ypotenuse

## Interactive Widget

Here is an interactive widget to help you learn about the cosine function on a right triangle.## What Is the Cosine Function?

The cosine function is a trigonometric function. The cosine of a given angle in a right triangle is the ratio of the length of the adjacent side to the length of the hypotenuse. The cosine function is defined by the formula:The image below shows what we mean by the given angle (labelled θ), the adjacent and the hypotenuse:

## How to Rearrange the Cosine Function Formula

A useful way to remember simple formulae is to use a small triangle, as shown below:Here, the

**C**stands for

**C**os θ, the

**A**for

**A**djacent and the

**H**for

**H**ypotenuse (from the

**CAH**in SOH

**CAH**TOA). To find the formula for the Adjacent, cover up the A with your thumb:

This leaves C

**next to**H - which means C

**times**H, or, Cos θ

**×**Hypotenuse. This tells you that:

Adjacent = Cos θ × Hypotenuse

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