(2mx2-x2+5x+8)-(7x2-3y+5x)
=2mx2-x2+5x+8-7x2+3y-5x
=(2m-1-7)x2+3y+8
因上式的值与x无关,即含x项的系数为0,有
2m-1-7=0 => m=4