Question

The value of $\cos (-{840}^{\circ })$ is

• A. $\frac{\sqrt{3}}{2}$
• B. $-\frac{1}{2}$
• C. $\frac{1}{\sqrt{2}}$
• D. $\frac{1}{2}$

Answer 1

The correct option is B $-\frac{1}{2}$

$\cos (-{840}^{\circ })=\cos (-{36}^{\circ }-{36}^{\circ }-{12}^{\circ })$
$=\cos (-{120}^{\circ })=\cos {120}^{\circ }=-\frac{1}{2}$