y=cosx
则√(1-y^2)=sinx
dy=-sinx
则sin2x=2sinxcosx=2y√(1-y^2)
原式=∫-(sinx)^2dx
=-∫(1-cos2x)/2 dx
=-1/4∫(1-cos2x)d(2x)
=-1/4(2x-sin2x)+C
=-[arccosy-y√(1-y^2)]/2+C