f(x) = sinx - cosx
= √2 sin(x - π/4)
0 ≤ x ≤ π/2
-π/4 ≤ x - π/4 ≤ π/4
-1/√2 ≤ sin(x - π/4) ≤ 1/√2
-1 ≤ √2 sin(x - π/4) ≤ 1
-1 ≤ f(x) ≤ 1
最小值：-1
最大值：1