sinx-cosx=2^(1/2)*{1/[2^(1/2)]*sinx-1/[2^(1/2)]*cosx}
=2^(1/2)*[cos(45+o)]
定义域:
0≤ sinx-cosx ≤ 2^(1/2)
画图求
值域:0≤f(x)≤(1/2)