## Precalculus (6th Edition) Blitzer

We have to solve this equation: \begin{align} & d=-4\cos \left( \frac{\pi }{3} \right)\times 2 \\ & d=-4\cos \left( \frac{2\pi }{3} \right) \\ & d=-4\cos \left( \pi -\frac{\pi }{3} \right) \\ & d=4\cos \frac{\pi }{3} \\ \end{align} Put $\cos \frac{\pi }{3}=\frac{1}{2}$ , \begin{align} & d=4\times \frac{1}{2} \\ & d=2 \\ \end{align} Thus, to find the value, we do not have to solve the trigonometric equation; we can find the value of the trigonometric angle and simplify it arithmetically.