Explanation
We are required to evaluate cos 120° without using a calculator.
This is achieved thus:
We know that 120° lies in the second quadrant.
We also know that cosine is negative in the second quadrant.
Therefore, we have:
[tex]\begin{gathered} \cos120\degree=\cos(180\degree-60\degree) \\ \Rightarrow-\cos60\degree \\ \text{ We know that }\cos60\degree=\frac{1}{2} \\ \\ \therefore-\cos60\degree=-\frac{1}{2} \end{gathered}[/tex]
Hence, the answer is:
[tex]-\frac{1}{2}[/tex]
Option B is correct.
