a+b+c=7 a=7-b-c b=7-a-c c=7-a-b
(7-a-b)/(a+b) +(7-b-c)/(b+c) +(7-a-c)/(a+c)
=7((1/a+b)+(1/b+c)+(1/c+a))-((a+b)/(a+b)+(b+c)/(b+c)+(c+a)/(c+a))
=10-1-1-1=7

厉害