y=sin(x^2)/(sinx)^2用对数求导简单：
lny=lnsin(x^2)-ln(sinx)^2
y'/y=2xcos(x^2)/sin(x^2)-2sinxcosx/(sinx)^2
=2xcot(x^2)-2cotx
所以：y'=y[2xcot(x^2)-2cotx]