Question

Find the value of
$\cos {270}^o$

Answer 1

$\cos {270}^{\circ }=\cos ({180}^{\circ }+{90}^{\circ })$

${180}^{\circ }+\theta$ is in third quadrant.By ASTC rule, $\cos \theta$ is negative in third quadrant.

Hence $\cos ({180}^{\circ }+{60}^{\circ })=\cos {90}^{\circ }=0$