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Mathematics

If x = 65\sqrt{6} - \sqrt{5}, then x1xx - \dfrac{1}{x} is equal to:

  1. 1

  2. 11

  3. 262\sqrt{6}

  4. -2 5\sqrt{5}

Rational Irrational Nos

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Answer

Given, x = 65\sqrt{6} - \sqrt{5}

1x=165=165×(6+5)(6+5)=6+5(6)2(5)2=6+565=6+51\Rightarrow \dfrac{1}{x} = \dfrac{1}{\sqrt{6} - \sqrt{5}}\\[1em] = \dfrac{1}{\sqrt{6} - \sqrt{5}} \times \dfrac{(\sqrt{6} + \sqrt{5})}{(\sqrt{6} + \sqrt{5})}\\[1em] = \dfrac{\sqrt{6} + \sqrt{5}}{(\sqrt{6})^2 - (\sqrt{5})^2}\\[1em] = \dfrac{\sqrt{6} + \sqrt{5}}{6 - 5}\\[1em] = \dfrac{\sqrt{6} + \sqrt{5}}{1}

Now,

x1x=65(6+51)=6565=25.\Rightarrow x - \dfrac{1}{x} = \sqrt{6} - \sqrt{5} - \Big(\dfrac{\sqrt{6} + \sqrt{5}}{1}\Big)\\[1em] = \sqrt{6} - \sqrt{5} - \sqrt{6} - \sqrt{5} \\[1em] = -2\sqrt{5}.

Hence, option 4 is correct option.

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