Near Twin Radicals

Algebra Level 3

x 1 x + 1 1 x = x \large \sqrt{ x - \dfrac1x } + \sqrt{1 - \dfrac1x} = x

Find all the real solutions to the equation above.

5 + 1 2 \frac{\sqrt5+1}2 5 1 4 \frac{\sqrt5-1}4 5 + 1 4 \frac{\sqrt5+1}4 All of these choices

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3 solutions

Chung Kevin
Feb 29, 2016

Since we're dealing with radicals, let's square both sides of the equation in hopes of converting the radical equation into a polynomial equation, which is easier to solve.

By squaring the equation in question, (recall that ( a + b ) 2 = a 2 + b 2 + 2 a b (a+b)^2=a^2+b^2+2ab ), we have

( x 1 x ) + ( 1 1 x ) + 2 ( x 1 x ) ( 1 1 x ) = x 2 x 2 + x 2 + 2 ( x 2 1 ) ( x 1 ) = x 3 ( 2 ( x 2 1 ) ( x 1 ) ) 2 = ( x 3 x 2 x + 2 ) 2 4 ( x 2 1 ) ( x 1 ) = ( x 3 x 2 x + 2 ) 2 x 6 2 x 5 x 4 + 2 x 3 + x 2 = 0 ( x 3 + x 2 + x ) 2 = 0 x ( x 2 x 1 ) = 0 x = 0 , 1 ± 5 2 by Quadratic formula \begin{aligned} \left( x- \dfrac1x\right) + \left( 1 - \dfrac1x \right) + 2 \sqrt{ \left( x - \dfrac1x \right) \left( 1 - \dfrac1x \right)} &=& x^2 \\ x^2 + x -2 + 2 \sqrt{(x^2-1)(x-1)} &= & x^3 \\ \left( 2 \sqrt{(x^2-1)(x-1)} \right)^2 &= & (x^3 -x^2 - x + 2)^2 \\ 4(x^2-1)(x-1) &=& (x^3 -x^2 - x + 2)^2 \\ x^6 -2x^5 - x^4+2x^3+x^2 &=& 0 \\ (x^3 +x^2 + x)^2 &=& 0 \\ x(x^2-x - 1) &=& 0 \\ x &=& 0, \dfrac{1\pm \;\sqrt5}2 \qquad \text{ by Quadratic formula} \end{aligned}

But substituting all these three values of x x into the original equation shows that x = 0 x=0 and x = 1 5 2 x = \dfrac{1-\sqrt5}2 are extraneous roots (they do not satisfy the given equation). Thus the solution of x x satisfying the equation in question is 1 + 5 2 \boxed{ \dfrac{1+\sqrt5}2 } .

Those are extraneous roots, there is a typo I think!! :p

Raushan Sharma - 5 years, 3 months ago

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Oh! Thank you for spotting my mistake! I've corrected it.

Chung Kevin - 5 years, 3 months ago
Lorenzo Moulin
Mar 3, 2016

汶良 林
Jan 9, 2017

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