Subjective Type

Show that the square of $$\dfrac{(\sqrt{26 - 15\sqrt{3}})}{(5\sqrt{2} - \sqrt{38 + 5\sqrt{3}})}$$ is a rational number.

Solution

Given,
$$ x = \dfrac{\sqrt{26 - 15\sqrt{3}}}{5\sqrt{2} - \sqrt{38 + 5\sqrt{3}}}$$

$$x^2 = \dfrac{26 - 15\sqrt{3}}{50 + 38 + 5\sqrt{3} - 10\sqrt{76 + 10\sqrt{3}}}$$

$$\Rightarrow x^2 = \dfrac{26 - 15\sqrt{3}}{88 + 5\sqrt{3} - 10\sqrt{75 + 1 + 10\sqrt{3}}}$$

$$\Rightarrow x^2 = \dfrac{26 - 15\sqrt{3}}{88 + 5\sqrt{3} - 10\sqrt{(5\sqrt{3} + 1)^2}}$$

$$\Rightarrow x^2 = \dfrac{26 - 15\sqrt{3}}{88 + 5\sqrt{3} - 10( 5\sqrt{3} + 1)}$$

$$\Rightarrow x^2 = \dfrac{26 - 15\sqrt{3}}{3(26 - 15\sqrt{3})} = \dfrac{1}{3}$$ which is a rational number.


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