令t=tanx
则1/(1+(sinx)²)=(1+t²)/(1+2t²)
dx=dt/(1+t²)
∫((sinx)²/(1+(sinx)²))dx=∫(1-1/(1+(sinx)²))dx
=x-∫(1/1+2t²)dt=x-(1/√2)arctan(√2t)+c
=x-(1/√2)arctan(√2(tanx))+c