M=2X的平方+4xy-2x-3;N=-x+xy+2,
3M+6N
=3(2x^2+4xy-2x-3)+6(-x+xy+2)
=6x^2+12xy-6x-9
.+6xy-6x+12
=6x^2+18xy-12x+3,
无法做到与x无关.
请检查题目.已知M=2X的平方+4xy-2x-3,N=-x的平方+xy+2M=2X的平方+4xy-2x-3;N=-x^2+xy+2，3M+6N=3(2x^2+4xy-2x-3)+6(-x^2+xy+2)=6x^2+12xy-6x-9.-6x^2+6xy.....+12=(18y-6)x+3,与x无关,∴18y-6=0,y=1/3.