1.设a/3=b/5=c/7=m
a=3m,b=5m,c=7m
3a+2b-4c=9
9m+10m-28m=9
m=-1
3a-2b-c=9m-10m-7m=-8m=8
2.
a/b=c/d=e/f=3/4
a=3/4*b
c=3/4*d
e=3/4*f
a+2c+e/b+2d+f=3/4(b+2d+f)/(b+2d+f)
=3/4
b+2d+f≠0,表示分母不为0,可以约去.
3.
设x/2=y/3=z/4＝m
x=2m,y=3m,z=4m
x+y-2z/2x-y
=(2m+3m-8m)/(4m-3m)
=-3