Question

How do you find the exact value for $\cos (240°)$?

Answer 1

Compute the value of the given expression:

Given: $\cos (240°)=\cos (270°-30°)$

using identity $\cos (270°-A)=-\sin \left(A\right)$

$$\begin{gathered} \cos (270°-30°)=-\sin (30°) \\ \Rightarrow \cos (240°)=-\frac{1}{2} \end{gathered}$$

Therefore, the value of the given expression is $-\frac{1}{2}$.

Hence, the exact value for $\cos (240°)$ is $-\frac{1}{2}$.