A/(x-2)-B/(x+1)
通分
=[A(x+1)-B(x-2)]/[(x-2)(x+1)]
=[(A-B)x+(A+2B)]/(x^2-x-2)
=(3x+4)/(x^2-x-2)
所以(A-B)x+(A+2B)=(3x+4)
所以A-B=3
A+2B=4
相减
2B+B=4-3
B=1/3,A=B+3=10/3
5A-B=50/3-1/3=49/3