3/3.5 + 3/3.7 + 3/3.9 + 3/9.11 + 3/11.13.
=3 . 2 . (1/3.5 + 1/3.7 + 3/3.9 + 1/9.11 + 1/11.13) : 2
=3 . (2/3.5 + 2/3.7 + 2/3.9 + 2/9.11 + 2/11.13) : 2
=3 . (1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13) : 2
=3 . (1/3 - 1/13) : 2
=3 . 10/39 : 2
= 10/13 : 2 = 5/13