y=cosx-(1/2)cos2x
=cosx-(2cos^2x-1)/2
=cosx-cos^2x+1/2
=-(cosx-1/2)^2+1/2+1/4
=-(cosx-1/2)^2+3/4
cosx=1/2 最大值=3/4
cosx=-1 最小值=-3/2
和=3/4-3/2=-3/4