解:将x=1,y=-2代入方程组ax+by=c,2ax-by=5c得:
a-2b=c.(1)
2a+2b=5c.(2)
(1)+(2)得:
3a=6c
a=2c
将a=2c代入(1)得:
2c-2b=c
2b=c
b=c/2
所以a:b:c=2c:c/2:c=4:1:2