Question

已知关于x,y的多项式ax²+2bxy+x²-x-2xy+y不含二次项，求5a-8b的值

Answer 1

ax²+2bxy+x²-x-2xy+y
=(a+1)x²+(2b-2)xy-x+y
前两项时二次项
所以系数为0
a+1=0,2b-2=0
a=-1,b=1
所以5a-6b=-5-6=-11

Answer 2

解:不含二次项,即二次项系数为零
ax²+2bxy+x²-x-2xy+y
=(ax²+x²)+(2bxy-2xy)+y-x
=(a+1)x²+(2b-2)xy-x+y
a+1=0,2b-2=0,a=-1,b=1
所以5a-8b=-13