1－2mxy－3y²＋4xy ＝1－3y²＋(-2m+4)xy
因为结果不含xy项,
则-2m+4=0
解得：m=2
所以,-m+2m²-m+(1/2)
=-2+2×2²-2+(1/2)
=-2+2×4-2+(1/2)
=-2+8-2+(1/2)
=9/2