2x²y与-1/2x的2m次方y的3n次方的和是单项式,则两个单项式是同类项,两个单项式中x、y分别对应的指数相等
2m=2 且 3n=1
m=1,n=1/3
4m²-6mn+7
=4×1²-6×1×1/3+7
=4-2+7
=9