(x-1)(x-2)分之2x+1=x-1分之A+x-2分之B 两边乘（x-1）(x-2)
2x+1=A(x-2)+B(x-1)
2x+1=(A+B)X-(2A+B)
A+B=2
2A+B=-1
两式相减
A=-1-2=-3
B=2-A=2+3=5