JAMB 2012 · UTME · Q35

If angle $\theta$ is $135^\circ$ , evaluate $\cos\theta$ .

Worked solution (try it first)

$135^\circ = 180^\circ - 45^\circ$ , so it is in the second quadrant with reference angle $45^\circ$ .
Cosine is negative in the second quadrant, so $\cos135^\circ = -\cos45^\circ$
$= -\frac{\sqrt2}{2}$ , option C.