已知m,n是系数且mx²+2xy-x与3x²-nxy+3y的差不含二次项求m²-2mn-2n²的值

已知m,n是系数且mx²+2xy-x与3x²-nxy+3y的差不含二次项求m²-2mn-2n²的值
已知m,n是系数且mx²+2xy-x与3x²-nxy+3y的差不含二次项求m²-2mn-2n²的值

已知m,n是系数且mx²+2xy-x与3x²-nxy+3y的差不含二次项求m²-2mn-2n²的值
(mx²+2xy-x)-(3x²-nxy+3y)
=mx²+2xy-x-3x²+nxy-3y
=(m-3)x²+(2+n)xy-x-3y
∵mx²+2xy-x与3x²-nxy+3y的差不含二次项
∴m-3=0
2+n=0
∴m=3
n=-2
∴m²-2mn-2n²
=9+12-8
=13