令 u=tanxx = arctanudx = 1/(1+u²) ducos²x = 1/(1+u²)∫ 1 / (1+cos²x) dx= ∫ 1/ [ 1 + 1/(1+u²) ] *1/(1+u²) du= ∫ 1/(2+u²) du= 1/√2 arctan(u/√2) +C= 1/√2...