(a+b)/ab=3
(b+c)/bc=4
(c+a)/ca=5
1/b + 1/a = 3 ------(1)
1/c + 1/b = 4 ------(2)
1/a + 1/c = 5 ------(3)
(1)-(2)-(3):
(1/b + 1/a)-(1/c + 1/b)-(1/a + 1/c) = -6
c = 1/3
1/a + 3 = 5
a = 1/2
3 + 1/b = 4
b = 1