(sinxcosx)′
=(sinx) ′cosx+sinx(cosx) ′
=cosxcosx+sinx(-sinx)
=(cosx)^2-(sinx)^2
=cos2x
求导法则：f(x)g(x)=f′(x)g(x)+f(x)g′(x)
(sinxcosx)′
=(sinx) ′cosx+sinx(cosx) ′
=cosxcosx+sinx(-sinx)
=(cosx)^2-(sinx)^2
=cos2x
f(x)g(x)=f′(x)g(x)+f(x)g′(x)
或者先用倍角公式sin2α=2sinα*cosα化简
(sinxcosx)′
=（1/2sin2x）′
=1/2cos2x*2
=cos2x
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