The Golden Equation

Algebra Level 3

1 + 1 1 + x + 1 1 + x = x + 1 x \sqrt{1+\frac{1}{1+x}}+\frac{1}{\sqrt{1+x}} = \sqrt{x}+\frac{1}{\sqrt{x}}

Which of the following is the root of the above equation?

Note: ϕ = 5 + 1 2 \phi=\frac{\sqrt5+1}{2} and denotes the golden ratio .

ϕ 2 \phi-2 ϕ 1 \phi-1 ϕ 3 \dfrac{\phi}{3} ϕ 2 \dfrac{\phi}{2} ϕ 2 \phi^2 None of the above

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1 solution

Rohit Udaiwal
Apr 5, 2016

We have 1 + 1 1 + x + 1 1 + x = x + 1 x x + 2 + 1 x + 1 = x + 1 x [ simplifying ] x ( x + 2 + 1 ) = ( x + 1 ) x + 1 x ( x + 3 + 2 x + 2 ) = ( x + 1 ) 3 [ ( x + 1 ) x + 1 = ( x + 1 ) 3 2 ] x 2 + 3 x + 2 x x + 2 = x 3 + 3 x 2 + 3 x + 1 x 3 + 2 x 2 2 x x + 2 + 1 = 0 ( x x + 2 ) 2 2 x x + 2 + 1 = 0 ( x x + 2 1 ) 2 = 0 x x + 2 = 1 x 2 ( x + 2 ) = 1 x 3 + 2 x 2 1 = 0 ( x + 1 ) ( x 2 + x 1 ) = 0 x = 1 , 1 ± 1 + 4 2 = 1 , 1 ± 5 2 \begin{aligned}\sqrt{1+\dfrac{1}{1+x}}+\dfrac{1}{\sqrt{1+x}} = & \sqrt{x}+\dfrac{1}{\sqrt{x}} \\ \dfrac{\sqrt{x+2}+1}{\sqrt{x+1}}= & \dfrac{x+1}{\sqrt{x}} \quad \quad & [\text{simplifying}]\\ \sqrt{x}(\sqrt{x+2}+1)= & (x+1)\sqrt{x+1} \\ x(x+3+2\sqrt{x+2})= & (x+1)^3 \quad \quad & [\because (x+1)\sqrt{x+1}=(x+1)^{\frac{3}{2}}] \\ x^2+3x+2x\sqrt{x+2}= & x^3+3x^2+3x+1 \\ x^3+2x^2-2x \sqrt{x+2}+1= & 0 \\ (x\sqrt{x+2})^2-2x\sqrt{x+2}+1= & 0 \\ (x\sqrt{x+2}-1)^2= & 0 \\ x\sqrt{x+2}= & 1 \\ x^2(x+2)= & 1 \\ x^3+2x^2-1= & 0 \\ (x+1)(x^2+x-1)= & 0 \\ x= & -1,\dfrac{-1\pm\sqrt{1+4}}{2} \\ = & -1,\dfrac{-1\pm\sqrt5}{2} \\ \end{aligned} But notice that 1 , 1 5 2 -1,\dfrac{-1-\sqrt5}{2} do not satisfy the above equation. x = 5 1 2 = ϕ 1 \therefore \begin{aligned} x= & \boxed{\dfrac{\sqrt5-1}{2}} \\ = & \phi-1 \end{aligned}

Golden solution... + ( ϕ 2 ϕ ) +(\phi^2-\phi) .....

PS:Correct the typo where you have found x.. You forgot to add a minus sign- 1 ± 5 2 \dfrac{\color{#D61F06}{-}1\pm\sqrt5}{2} .

Rishabh Jain - 5 years, 2 months ago

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Edited.Thanks for your Golden words :P

Rohit Udaiwal - 5 years, 2 months ago

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