因为b+1/c=1,c+1/a=1
所以b=1-1/c=c-1/c ,1/b=c/c-1 ,a=1/1-c
因此ab+1/b=(1/1-c)*(c-1/c)+c/c-1=-1/c+c/c-1=1+1/(c^2-c)