Find the square root of 170 - 20\(\sqrt{30}\)
 

  • A 2 \(\sqrt{10}\) - 5\(\sqrt{3}\)
  • B 2 \(\sqrt{5}\) - 5\(\sqrt{6}\)
  • C 5 \(\sqrt{10}\) - 2\(\sqrt{3}\)
  • D 3 \(\sqrt{5}\) - 8\(\sqrt{6}\)

The correct answer is B. 2 \(\sqrt{5}\) - 5\(\sqrt{6}\)

\(\sqrt{170 - 20 \sqrt{30}} = \sqrt{a} - \sqrt{b}\)

Squaring both sides,

\(170 - 20\sqrt{30} = a + b - 2\sqrt{ab}\)

Equating the rational and irrational parts, we have

\(a + b = 170 ... (1)\)

\(2 \sqrt{ab} = 20 \sqrt{30}\)

\(2 \sqrt{ab} = 2 \sqrt{30 \times 100} = 2 \sqrt{3000} \)

\(ab = 3000 ... (2)\)

From (2), \(b = \frac{3000}{a}\)

\(a + \frac{3000}{a} = 170 \implies a^{2} + 3000 = 170a\)

\(a^{2} - 170a + 3000 = 0\)

\(a^{2} - 20a - 150a + 3000 = 0\)

\(a(a - 20) - 150(a - 20) = 0\)

\(\text{a = 20 or a = 150}\)

\(\therefore b = \frac{3000}{20} = 150 ; b = \frac{3000}{150} = 20\)

\(\sqrt{170 - 20\sqrt{30}} = \sqrt{20} - \sqrt{150}\) or \(\sqrt{150} - \sqrt{20}\)

= \(2\sqrt{5} - 5\sqrt{6}\) or \(5\sqrt{6} - 2\sqrt{5}\)

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