-1≤sin(2x-π/4)≤1
-√2≤√2sin(2x-π/4)≤√2
值域【-√2,√2】
0≤x≤π/2
0≤2x≤π
-π/4≤2x-π/4≤3π/4
2x-π/4=-π/4时f(x)取最小值f(x)min=√2sin(-π/4) = √2*(-√2/2) = -1