axb=30....1bxc=35.....2cxa=42......31/2得 a/c=30/35=6/71/3得 b/c=30/42=5/72/3得 b/a=35/42=5/6所以，a=6 b=5 c=7 a+b+c=18
a = 6b = 5c = 7
5+6+7＝18
