Question

use a reference angle to find the exact value of the following expression. do not use a calculator.
cos (225°)

determine the reference angle for 225°.
the reference angle is □°.

Explanation:

Step1: Identify the quadrant of \(225^\circ\)

\(225^\circ\) lies in the third quadrant because \(180^\circ<225^\circ<270^\circ\).

Step2: Calculate the reference angle

For an angle \(\theta\) in the third quadrant, the reference angle \(r\) is given by \(r = \theta - 180^\circ\).
Substituting \(\theta = 225^\circ\), we get \(r=225^\circ - 180^\circ = 45^\circ\).

Answer:

\(45\)