三分之X=四分之Y=五分之Zx：y：z=3:4:5设x=3a y=4a z=5a(x+y)/(x-2y+3z)=(3a+4a)/(3a-8a+15a)=7a/10a=7/10
设x/3=y/4=z/5=k，则：x=3k，y=4k，z=5k；代入下式(x+y)/(x-2y+3z)=(3k+4k)/(3k-8k+15k)=7/10
