A4. How many degrees is the angle $\alpha$ in a triangle if $\cos \alpha=-\frac{1}{2}$?
(A) $60^{\circ}$
(B) $30^{\circ}$
(C) $120^{\circ}$
(D) $150^{\circ}$
(E) $30'$
A4. Since the value of the cosine is negative, the solution is an obtuse angle. We know that $\cos 120^{\circ}=-\frac{1}{2}$.