设a+b=9k,
b+c=5k
c+a=6k
a=5k,b=4k,c=k
(1)a:b:c=5k:4k:k=5:4::1
(2)(a-b+c)/(2a+b)
=(5k-4k+k)/(10k+4k)
=2k/(14k)
=1/7