Which of the following numbers

MATHEMATICS · Single correct · AoPS · Mathematics · Special Factorizations · Special Factorizations

Problem

Which of the following numbers, when multiplied by

(\sqrt{11}+\sqrt{7})

, will give a rational result? (a)

\sqrt{11}+\sqrt{5}

(b)

\sqrt{11}+\sqrt{7}

(c)

\sqrt{11}-\sqrt{7}

(d)

\sqrt{7}-\sqrt{5}