Four number pairs are given, out of which three are same in a certain way. Find the odd pair that does not belong to the group?

(81 : 9 : 19)

\(\sqrt {64}\) + \(\sqrt {100}\) = 8 + 10 = 18

\(\sqrt {25}\) + \(\sqrt {16}\) = 5 + 4 = 9

\(\sqrt {16}\) + \(\sqrt {64}\) = 4 + 8 = 12

\(\sqrt {81}\) + \(\sqrt {9}\) = 9 + 3 = 12 \(\neq \) 19