cscx=1/sinx
(cscx)'=(1/sinx)'
=1/sin^2(x)*(sinx)' ,(这一步错了,应该是：-1/sin^2(x)*(sinx)'
=1/sin^（x)*cosx
=cotx*1/sinx
=cotx*cscx
注：(1/x)'=-1/x^2