cos(120°+x)
=cos120°cosx-sin120°sinx
=-cos60°cosx-sin60°sinx
=-1/2 cosx-√3/2 sinx
同理,
cos(120°-x)=-1/2 cosx+√3/2 sinx
所以原式
=cosx-1/2 cosx-1/2 cosx
=0