:ax的平方+2bxy-x的平方-3X+2XY+Y不含2次项,求3a-4b的值
ax^2+2bxy-x^2-3x+2xy+y
=(a-1)x^2+(2b+2)xy-3x+y
不含二次项,说明二次项的系数为0
所以a-1=0,2b+2=0
=>a=1,b=-1
3a-4b
=3*1-4*(-1)
=3+4
=7