Question

If $0<\theta <360^{\circ }$, then solutions of $\cos \theta =-1/2$ are

• A. $120^{\circ },360^{\circ }$
• B. $240^{\circ },90^{\circ }$
• C. $60^{\circ },270^{\circ }$
• D. $120^{\circ },240^{\circ }$

Answer 1

Answer: (D)

$\cos \theta =-1/2=\cos 120^{\circ }$ or $\cos 240^{\circ },\left[0<\theta <360^{\circ }\right]$
$\therefore \theta =120^{\circ },240^{\circ }$