y=cos^2x-2cos^2(x/2)
=cos^2x-cosx-1
=(cosx-1/2)^2-5/4
一个单调增区间[-π/3,0]答案是（π/3,π）(0,π/3]单减[π/3,π/2]单增[π/2,π]单增[π,5π/3]单减[5π/3,2π]单增应该还有个定义域限制