积化和差公式
f(x)=-(cos2x-cosπ/2)+sin2x
=sin2x-cos2x
=根号2*[sin2x*cosπ/4-cos2x*sinπ/4]
=根号2 *sin(2x-π/4)
所以,f(x)∈[-根号2,根号2]
最大值 根号2