这个不用二类换元法反而简单些吧设x=sint，dx=costdt，√(1-x^2)=cost原式=∫sin²t/cost*costdt=∫sin²tdt=∫(1-cos2t)/2dt=1/2∫dt-1/2∫cos2tdt=t/2+1/4∫cos2td2t=t/2+1/4*sin2t+C=t/2+1/2*sintcost+C=1/2*arcsinx+1/2*x/√(1-x^2)+C
