What is \(\sqrt{17 - 4\sqrt{15}} + \sqrt{8 - 2\sqrt{15}}\) equal to?

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UPSC CDS-I 2025 (Elementary Mathematics) Official Paper (Held On: 13 Apr, 2025)
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  1. \(\sqrt{3} \)
  2. \(2\sqrt{3}\)
  3. \(2\left(\sqrt{5} - \sqrt{3}\right)\)
  4. \(2\left(\sqrt{5} + \sqrt{3}\right)\)

Answer (Detailed Solution Below)

Option 1 : \(\sqrt{3} \)
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Detailed Solution

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Given:

The required expression: \(\sqrt{17 - 4\sqrt{15}} + \sqrt{8 - 2\sqrt{15}}\)

Calculation:

\(\sqrt{17 - 4\sqrt{15}}\) = \(\sqrt{(2\sqrt3)^2 + (\sqrt5)^2 - 2 \times 2\sqrt{3}\times \sqrt5}\) = \(\sqrt{(2\sqrt3 -\sqrt 5)^2}\) = \(2\sqrt3 - \sqrt5\)

\(\sqrt{8 - 2\sqrt{15}}\) = \(\sqrt{(\sqrt{3})^2 + (\sqrt{5})^2 - 2 \times \sqrt{3} \times \sqrt{5}}\) = \(\sqrt{(\sqrt{5} -\sqrt{3})^2 }\) = \(\sqrt{5} - \sqrt{3}\)

Now, adding both terms:

\(2\sqrt3 - \sqrt5\) + \(\sqrt{5} - \sqrt{3}\) = \(\sqrt3\)

Therefore, the final answer is option 1.

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