f(x)=sinxcosx-(sinx+cosx)
=0.5[1-(sinx+cosx)]²-1
sinx+cosx=根号2sin(x+π/4)
函数f(x)在 (0,π/2）上的单调递增区间是(π/4,π/2）若函数f(x)在区间（π/2,π）上是单调递减函数，求实数m的取值范围