Question

find the exact value of the expression

$\cos (-120°)$

Short answer

$\cos \left(120°\right)=-\frac{1}{2}$

Step-by-step solution

Step 1. Rewriting the expression 

As cos is a even function so, cos(x)=cos(-x)

So given expression can be written as $\cos (120°)$

Step 2. Calculations

$$\begin{gathered} \cos (\mathrm{\pi }-\mathrm{x})=-\cos \left(\mathrm{x}\right) \\ \cos (120°)=\cos \left(\mathrm{\pi }-60°\right)=-\cos \left(60\right)° \\ -\cos \left(60\right)°=-\frac{1}{2} \end{gathered}$$