f(x)=2sinxcosx-2(cosx)^2
=sin2x-(1+cos2x)
=sin2x-cos2x-1
=√2sin(2x-π/4)-1
π/8≤x≤3π/4
π/4≤2x≤3π/2
0≤2x-π/4≤5π/4
函数sint在[0,5π/4]上的最大值为1,最小值为 （-√2/2)
-√2/2≤sin(2x-π/4)≤1
-1≤√2sin(2x-π/4)≤√2
-2≤√2sin(2x-π/4)-1≤√2-1
所以,
f(max)=√2-1
f(min)=-2