### Home > PC > Chapter 6 > Lesson 6.2.2 > Problem6-87

6-87.

Use the angle difference formula for cosine to simplify $\cos \left(π − θ\right)$. Write your answer as an identity and check it for $θ = \frac { \pi } { 3 }$.

\begin{aligned}\cos \left(π − θ\right) &= \cos π \cos θ + \sin π \sin θ\\ &= −1\cos θ + 0 · \sin θ\\ \cos \left(π − θ\right) &= −\cos θ\end{aligned}