(sin²x)' = 2sinx(sinx)' = 2sinxcosx = sin2x或：(sin²x)' = [(1-cos2x)/2]' = [1/2 - (cos2x)/2]' = 0 - ½(-sin2x)(2x)' = ½(sin2x)×2 = sin2x
sin(2x)的导数为2cos(2x)sinx的平方的导数为2sinxcosx
y＝cos.2
