For $a$, we have:
\[a^2 = (\sqrt{13}+\sqrt{11})^2 = 13 + 11 + 2\sqrt{13}\sqrt{11} = 24 + 2\sqrt{143}\]
For $b$, we have:
\[b^2 = (\sqrt{15}+\sqrt{9})^2 = 15 + 9 + 2\sqrt{15}\sqrt{9} = 24 + 2\sqrt{135}\]
For $c$, we have:
\[c^2 = (\sqrt{18}+\sqrt{6})^2 = 18 + 6
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