原式＝cos(x+2x)=cosxcos2x-sinxsin2x=cosx((cosx)^2-(sinx)^2)-2(sinx)^2cosx
=(cosx)^3-3cosx(sinx)^2
=cosx(1-4(sinx)^2)
cosx=0 或者(sinx)^2=1/4
x=（k+1/2）pi 或者x=（k/2+1/4）pi k属于Z