已知a:b:c=2:3:7，且a-b+c=12，求2a+b-3c的值

a:b:c=2:3:7
a/2=b/3=c/7
令他们a/2=b/3=c/7=t
=>
a=2t,b=3t,c=7t
a-b+c=12
2t-3t+7t=12
t=2

2a+b-3c
=4t+3t-21t
=-14t
=-28

a=2 b=3 c=7
2a+b-3c=-14

12/(2+3+7)=1,a=2,b=3,c=7

2a+b-3c=4+3-21=-14

a=2k,b=3k,c=7k
a-b+c=2k-3k+7k=6k=12
k=2
2a+b-3c=4k+3k-21k=-14k=-28

设a=2x,b=3x,c=7x.
所以a-b+c=12 即2x-3x+7x=12 x=2
结果2a+b-3c=4x+3x-21x=-14x=-28