b+1/c=1
b=1-1/c=（c-1）/c
c+1/a=1
ac+1=a
a=1/（1-c）
（ab+1）/b
={[1/(1-c) · (c-1)/c]+1 }/[(c-1)/c]
=[(c-1）/c]·[（c/(1-c)]
=-1