恒等式cos2x = 2cos²x - 1
所以cos²x = (1 + cos2x)/2
∫ (cost)² dt
= ∫ (1 + cos2t)/2 dt
= (1/2)∫ dt + (1/2)(1/2)∫ cos2t d(2t)
= (1/2)t + (1/4)sin2t + C