جذر عدد \(10 + 2\sqrt {21} \) کدام است؟
\(\sqrt 7 + \sqrt 5 \)
\(\sqrt 7 - \sqrt 5 \)
\(\sqrt 7 + \sqrt 3 \)
\(\sqrt 7 - \sqrt 3 \)
\(\begin{array}{l}\sqrt {10 + 2\sqrt {21} } = \sqrt {7 + 3 + 2\sqrt {7 \times 3} } = \\\\\sqrt {{{(\sqrt 7 + \sqrt 3 )}^2}} = \sqrt 7 + \sqrt 3 \end{array}\)