# Using the Tangent Function to Find the Adjacent

## Using the Tangent Function to Find the Adjacent of a Right Triangle

The tangent function relates a given angle to the opposite side and adjacent side 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 opposite is the length of the side opposite the angle and the adjacent is the length of side next to the angle. The image below shows what we mean:

## How to Use the Tangent 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 opposite.

### Question

What is the length of the adjacent of the right triangle shown below?

### Step-by-Step:

# 1

Start with the formula:

Adjacent = opposite / tan θ

**Don't forget:** / means ÷

# 2

Substitute the angle θ and the length of the opposite into the formula. In our example, θ = 45° and the opposite is 3 cm.

Adjacent = 3 / tan (45°)

Adjacent = 3 ÷ tan (45°)

Adjacent = 3 ÷ 1

Adjacent = 3

### Answer:

The length of the adjacent of a right triangle with an angle of 45° and an opposite of 3 cm is 3 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 **O**pposite.

The two letters we are looking for are **OA**, which comes in the **TOA** in SOH CAH **TOA**.

This reminds us of the equation:

**T**an θ = **O**pposite / **A**djacent

This is rearranged to get the formula at the top of the page (see **Note**).

**A**djacent = **O**pposite / **T**an θ