求导?
y=sin2xcosx
y'=(sin2x)'*cosx+sin2x*(cosx)'
=cos2x*(2x)'*cosx+sin2x(-sinx)
=2cos2xcosx-sin2xsinx
y=lncosx
y'=(1/cosx)*(cosx)'
=(1/cosx)*(-sinx)
=-tanx