f(x)=根号下2sinxsin(x-π/4)-1/2=根号下2sinx×(根号下2sinx/2-根号下2cosx/2)-1/2=sin²x-sinxcosx-1/2=-cos2x/2-sin2x/2=-根号下2sin(2x+π/4)/2f(A)=-根号下2sin(2A+π/4)/2A为锐角三角形最小角所以 A...前面系数是负数，那只能取得最小值对 π/4<2A+π/4<=11π/12 当2A+π/4=11π/12时 有最大值 f(A)=(-根号下2/2)×1/2=-根号下2/4