1. cos3x-cosx=cos(2x+x)-cos(2x-x)
=cos2xcosx-sin2xsinx-[cos2xcosx+sin2xsinx]
=-2sin2xsinx=-4sin^2x*cosx
2.cos3x=cosx-4sin^2xcosx
cos3x+2cosx=0
cosx-4sin^2xcosx +2cosx=0
cosx(3-4sin^2x)=0
cosx=0x=kπ+π/2
3-4sin^2x=0
sinx=√3/2x=2kπ+π/3或 x=2kπ+2π/3
sinx=-√3/2x=2kπ-π/3或 x=2kπ-2π/3