Question 19
(IRE 5) Let $a$ be a rational number with $0\lt a\lt 1$ and suppose that
$$\cos3\pi a+2\cos2\pi a=0.$$
(Angle measurements are in radians.) Prove that $a=2/3$.
$$\cos3\pi a+2\cos2\pi a=0.$$
(Angle measurements are in radians.) Prove that $a=2/3$.