Find all of the solutions of th equation cosx+cos2x=0

题目

Find all of the solutions of th equation cosx+cos2x=0

答案

Solution:
From cos2x=2cos^2 x-1,we have
cosx+cos2x=2cos^2 x-1+cosx=0.
Thus (cos x+1)(2cos x-1)=0.
So cos x=-1 or cos x=1/2
.(esay)