求cosx+cos(120-x)+(120+x)的值

题目

求cosx+cos(120-x)+(120+x)的值
快

答案

=cosx+cos120cosx+sinxsin120+cos120cosx-sinxsin120
=cosx+2cos120cosx
=cosx+2*(-cos60)cosx
=cosx-cosx=0