4c9的精选

问题详情：若a:b:c=3:5:7,且3a+2b-4c=9,则a+b+c的值等于（ ）A.-3              B.-5              C.-7               D.-15【回答】D【解析】已知a:b:c=3:5:7，设a=3k，b=5k，c=7...