sin²x=(1-cos2x)/2所以是x/2-1/4*sin2x+CC是任意常数
y=∫sin^2xdx=∫(sinx)^2dx=∫(1-cos2x)dx/2=∫(1/2)dx-∫cos2xdx/2=(1/2)x-∫cos2xd2x/4=(1/2)x-(1/4)sin2x+c.
