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I was playing around with Desmos Graphing Calculator and found that the two graphs below intersect at (-1,0), (0,-1), (1.618, 1.618), and (-0.618, -0.618). The latter two points are the golden ratio and the negative of the golden ratio plus 1.

Graphs:

  • $y = x^2 - 1$
  • $x = y^2 - 1$

Why do the graphs intersect at these points?

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1 Answer 1

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You have $x = (x^2-1)^2 -1 \iff x+1 = (x^2-1)^2 \iff x^4 - 2x^2 - x = 0$ so $$x(x^3 - 2x - 1) = x(x+1)(x^2-x-1) =0$$ so the roots are $x=0,-1, \phi, -\phi^{-1}$ by using the quadratic formula on the last term.

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  • $\begingroup$ Is the last term ($−ϕ^−1$) is -0.618...? The properties of the golden ratio keep surprising me! $\endgroup$
    – Tim K
    Commented Feb 26, 2017 at 6:48
  • $\begingroup$ @TimK It's the negative of the inverse of $\phi$ $\endgroup$
    – Omry
    Commented Feb 26, 2017 at 6:49
  • $\begingroup$ @Omry, yes I understand the notation. I was just surprised. $\endgroup$
    – Tim K
    Commented Feb 26, 2017 at 6:51
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    $\begingroup$ @TimK $1+\frac{1}{\phi}=\phi$ $\endgroup$
    – Omry
    Commented Feb 26, 2017 at 6:52
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    $\begingroup$ @TimK That last property is just what you get from taking $x^2-x-1=0$, transferring the $-x-1$ to the other side of the equation, and dividing by x. Nothing special about $\phi$ in this one. $\endgroup$
    – Omry
    Commented Feb 26, 2017 at 6:59

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