y=sin2x+sinx-cosx=2sinxcosx-1+sinx-cosx+1=-（sinx-cosx）²+sinx-cosx+1
=-{（根号2sin（x-π/4）-1/2}²+5/4
sin（x-π/4）=根号2/4时.y取得最大值为5/4
函数y=sin2x+sinx-cosx的最大值为5/4