∫ cos²3t dt= (1/2)∫ (1 + cos6t) dt= (1/2)∫ dt + (1/2)∫ cos6t dt= (1/2)t + (1/2)(1/6)∫ cos6t d(6t)= (1/2)t + (1/12)sin6t + C两倍角公式：cos2x = 2cos²x - 1 =>

cos²x = (1/2)(1 + cos2x)，将x变为3t就可以了。