An algebra problem by Abhiram Rao

Algebra Level 3

If x = 3 + 2 3 2 x = \dfrac{\sqrt3 + \sqrt2}{\sqrt3- \sqrt2} and y = 3 2 3 + 2 y = \dfrac{\sqrt3 - \sqrt2}{\sqrt3 + \sqrt2} , then find the value of x 2 + y 2 x^2+y^2 .


The answer is 98.

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2 solutions

Chew-Seong Cheong
Jul 14, 2016

x + y = 3 + 2 3 2 + 3 2 3 + 2 = ( 3 + 2 ) 2 + ( 3 2 ) 2 ( 3 + 2 ) ( 3 2 ) = 5 + 2 6 + 5 2 6 1 = 10 ( x + y ) 2 = x 2 + y 2 + 2 x y 1 0 2 = x 2 + y 2 + 2 × 3 + 2 3 2 × 3 2 3 + 2 100 = x 2 + y 2 + 2 x 2 + y 2 = 100 2 = 98 \begin{aligned} x+y & = \frac {\sqrt 3 + \sqrt 2}{\sqrt 3 - \sqrt 2} + \frac {\sqrt 3 - \sqrt 2}{\sqrt 3 + \sqrt 2} \\ & = \frac {(\sqrt 3 + \sqrt 2)^2 + (\sqrt 3 - \sqrt 2)^2}{(\sqrt 3 + \sqrt 2)(\sqrt 3 - \sqrt 2)} \\ & = \frac {5+2\sqrt 6 + 5-2\sqrt 6}{1} \\ & = 10 \\ (x+y)^2 & = x^2 + y^2 + 2xy \\ 10^2 & = x^2 + y^2 + 2 \times \frac {\sqrt 3 + \sqrt 2}{\sqrt 3 - \sqrt 2} \times \frac {\sqrt 3 - \sqrt 2}{\sqrt 3 + \sqrt 2} \\ 100 & = x^2 + y^2 + 2 \\ \implies x^2 + y^2 & = 100 -2 = \boxed{98} \end{aligned}

Typo: a a and b b should be x x and y y . :)

A Former Brilliant Member - 4 years, 11 months ago

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Thanks. I have changed them.

Chew-Seong Cheong - 4 years, 11 months ago
Hung Woei Neoh
Aug 3, 2016

x = 3 + 2 3 2 = ( 3 + 2 ) ( 3 + 2 ) ( 3 2 ) ( 3 + 2 ) = ( 3 + 2 ) 2 3 2 = 5 + 2 6 x = \dfrac{\sqrt{3}+\sqrt{2}}{\sqrt{3}-\sqrt{2}}\\ =\dfrac{(\sqrt{3}+\sqrt{2})(\sqrt{3}+\sqrt{2})}{(\sqrt{3}-\sqrt{2})(\sqrt{3}+\sqrt{2})}\\ =\dfrac{(\sqrt{3}+\sqrt{2})^2}{3-2}\\ =5+2\sqrt{6}

y = 3 2 3 + 2 = ( 3 2 ) ( 3 2 ) ( 3 + 2 ) ( 3 2 ) = ( 3 2 ) 2 3 2 = 5 2 6 y=\dfrac{\sqrt{3}-\sqrt{2}}{\sqrt{3}+\sqrt{2}}\\ =\dfrac{(\sqrt{3}-\sqrt{2})(\sqrt{3}-\sqrt{2})}{(\sqrt{3}+\sqrt{2})(\sqrt{3}-\sqrt{2})}\\ =\dfrac{(\sqrt{3}-\sqrt{2})^2}{3-2}\\ =5-2\sqrt{6}

x 2 + y 2 = ( 5 + 2 6 ) 2 + ( 5 2 6 ) 2 = 25 + 4 ( 6 ) + 20 6 + 25 + 4 ( 6 ) 20 6 = 25 + 24 + 25 + 24 = 98 x^2+y^2\\ =(5+2\sqrt{6})^2+(5-2\sqrt{6})^2\\ =25+4(6)\color{#D61F06}{+20\sqrt{6}}+25+4(6)\color{#D61F06}{-20\sqrt{6}}\\ =25+24+25+24\\ =\boxed{98}

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