积分部分=2/[(cosx/sinxcosx)+(sin^2x/cosxsinx)]=2/[(1/sinx)+(sinx/cosx]=2/(cscx+tanx)
变形=2sinxcosx/[cosx+(1-cos2x)/2]dx=-2cosx[cosx+(1-cos2x)/2]d(cosx)令t=cosx
