已知 3X=啊vf fg其中AB是常数 则AB的值为

(3x+4)/(x^2-x-2) = A/(x-2)-B/(x-1) ,
B/(x-1) = A/(x-2)-(3x+4)/(x^2-x-2) = [A(x+1)-(3x+4)]/(x^2-x-2) ,
B(x^2-x-2) = A(x+1)(x-1)-(3x+4)(x-1) ,
Bx^2-Bx-2B = (A-3)x^2-x-(A-4) ,
对比同类项系数可得：
B = A-3 ,B = -1 ,2B = A-4 ,
解得：A = 2 ,B = -1 ；
所以,4A-B = 9 ；
选 B .