Question

已知关于x、y的多项式6mx2-8nxy+3x-4xy-x2+y-3中不含二次项，求整式3m-4n的值

Answer 1

关于x、y的多项式6mx²-8nxy+3x-4xy-x2+y-3中不含二次项，
所以二次项的系数为0.
即6m-1=0 8n+4=0
m=1/6 n=-1/2
3m-4n=3*1/6-（-1/2）*4
=1/2+2
=5/2

Answer 2

6mx²-8nxy+3x-4xy-x²+y-3

=（6m-1）x²-（8n+4）xy+3x+y-3

∵多项式中不含二次项

∴6m-1=0，8n+4=0

∴m=1/6，n=-1/2

∴3m-4n=5/2

Answer 3

∵6mx²-8nxy+3x-4xy-x²+y-3=（6m-1）x²-（8n+4）xy+3x+y-3

又∵多项式中不含二次项，只能是带有二次项的系数为0

∴6m-1=0，8n+4=0

∴m=1/6，n=-1/2

∴3m-4n=5/2

Answer 4

二次项的系数为0.
即6m-1=0 8n+4=0
m=1/6 n=-1/2
3m-4n=3*1/6-（-1/2）*4
=1/2+2
=5/2