f(X)=cos立方X+sin平方X-cosX
=cos立方X+1-cos平方X-cosX
cosx=t属于[-1,1]
f(X)=t^3-t^2-t+1
求导f'=3t^2-2t-1=(3t+1)(t-1)
[-1,-1/3] 递增[-1/3,1]递减
最大值时t=-1/3
代入
f(X)max=32/27