Question

选择题 已知3x+4/x²-x-2=A/x-2-B/x+1,其中A,B为常数,则4A-B的值为

A.13 B.9 C.7 D.5

Answer 1

A/(x-2)-B/(x+1)
=(A(x+1)-B(x-2))/(x-2)(x+1)
=((A-B)x+A+2B)/(x-2)(x+1)
3x+4=(A-B)x+(A+2B)
3=A-B(1)
4=A+2B(2)
(2)-(1) 1=3B
B=1/3
A=3+B=3+1/3=10/3
4A-B=4*10/3-1/3=40/3-1/3=39/3=13
A