16 + 1.25cos4a - 4cos2a = 16 + 1.25(2cos^2(2a) - 1) - 4cos2a;
t = cos2a, -1 <= t <= 1;<br>16 + 2.5t^2 -1.25 - 4t = 2.5t^2 - 4t + 14.75 = f(t);
f'(t) = 5t - 4; tmin = 4/5; -1 <= 4/5 <= 1 => fmin = f(tmin) = 2.5 * 16/25 - 4 * 4/5 + 14.75 = 13.15 ─ ответ.