Using the Tangent Function to Find the Adjacent
Using the Tangent Function to Find the Adjacent 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.
What is the length of the adjacent of the right triangle shown below?
Start with the formula:
Adjacent = opposite / tan θ
Don't forget: / means ÷
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
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 SOH CAH TOA.
Looking at the example above, we are trying to find the Adjacent and we know the Opposite.
The two letters we are looking for are OA, which comes in the TOA in SOH CAH TOA.
This reminds us of the equation:
Tan θ = Opposite / Adjacent
This is rearranged to get the formula at the top of the page (see Note).
Adjacent = Opposite / Tan θ
The slider below gives another example of finding the adjacent of a right triangle (since the angle and opposite are known).Open the slider in a new tab