<s>[INST] The locus of the intersection point of two perpendicular tangents to the curve $xy^2=1$. Find the locus of the intersection point of two perpendicular tangents to the curve $xy^2=1$.
I find out that the tangents to this curve with slope $m$ has this general form:
$y = mx+(-2m)^{\frac{1}{3}}-m \left( -\frac{1}{2m} \right)^{\frac{2}{3}}$
The perpendicular tangent would have this form then:
$y = -\frac{1}{m}x+ \left(\frac{2}{m} \right)^{\frac{1}{3}}+\frac{1}{m} \left( \frac{m}{2} \right)^{\frac{2}{3}}$
I solved geometrically that the locus is the circle: $2x^2+2y^2-3\sqrt[3]{2}x=0$ (you can check it here: desmos.com/calculator/xt2xquh72l), but I couldn't solve it algebraically. I wonder if you could help me in eliminating $m$ in this system:
$\begin{cases}
y &= mx+(-2m)^{\frac{1}{3}}-m \left( -\frac{1}{2m} \right)^{\frac{2}{3}} \\
y &= -\frac{1}{m}x+ \left(\frac{2}{m} \right)^{\frac{1}{3}}+\frac{1}{m} \left( \frac{m}{2} \right)^{\frac{2}{3}} \\
\end{cases}
$
 [/INST] The parametric equations for the curve $xy^2=1$ could be $\forall \ t \neq 0$:
$\begin{cases}
x =& \displaystyle t^2 \\
y =& \displaystyle \frac{1}{t} \\
\end{cases}$
The tangent at the point $\left( t^2, \frac{1}{t} \right)$ is:
$m = \displaystyle \frac{\mathrm{d} y}{\mathrm{d}x} = \frac{ \frac{\mathrm{d}y}{\mathrm{d}t} }{ \frac{\mathrm{d}x}{\mathrm{d}t} } = \frac{ -\frac{1}{t^2} }{ 2t } = -\frac{1}{2t^3}$
The tangent equation to the curve in the point $\left( t^2, \frac{1}{t} \right)$ passing through $(x,y)$ is then:
$y - \frac{1}{t} = -\frac{1}{2t^3} \left( x-t^2 \right) \implies 2yt^3-3t^2+x=0$
Using Vieta's formulas to the previous cubic equation la anterior in $t$, their roots $t_1$, $t_2$ y $t_3$ satisfy:
$\begin{cases}
t_1+t_2+t_3 &= \frac{3}{2y} \\
t_1t_2+t_2t_3+t_1t_3 &= 0 \\
t_1t_2t_3 &= -\frac{x}{2y}
\end{cases}$
If we want two of these tangents to be perpendicular, the condition $m_1 m_2=-1$ must be satified, where $m_1$ and $m_2$ are their slopes. Then:
$\begin{array}{ll}
& \displaystyle m_1 \cdot m_2 = -1 \implies \left( -\frac{1}{2t_1^3} \right) \left( -\frac{1}{2t_2^3} \right) = \frac{1}{4t_1^3t_2^3}=-1 \implies 4t_1^3t_2^3 =-1 \implies \\
& \displaystyle t_1t_2 = -\frac{1}{\sqrt[3]{4}}
\end{array}$
Substituting this value in Vieta's formulas:
$\begin{array}{ll}
& \displaystyle \begin{cases}
t_1+t_2+t_3 &= \frac{3}{2y} \\
-\frac{1}{\sqrt[3]{4}}+t_2t_3+t_1t_3 &= 0 \\
-\frac{1}{\sqrt[3]{4}} t_3 &= -\frac{x}{2y}
\end{cases} \implies
\begin{cases}
t_1+t_2+\frac{\sqrt[3]{4}x}{2y} &= \frac{3}{2y} \\
-\frac{1}{\sqrt[3]{4}}+\frac{\sqrt[3]{4}x}{2y}t_2+\frac{\sqrt[3]{4}x}{2y}t_1 &= 0 \\
t_3 &= \frac{\sqrt[3]{4}x}{2y}
\end{cases} \implies \\
& \displaystyle \begin{cases}
t_1+t_2 &= \frac{3- \sqrt[3]{4}x}{2y} \\
\frac{\sqrt[3]{4}x}{2y} \left( t_1 +t_2 \right) &= \frac{1}{\sqrt[3]{4}} \\
\end{cases} \implies \frac{\sqrt[3]{4}x}{2y} \cdot \frac{3- \sqrt[3]{4}x}{2y} = \frac{1}{\sqrt[3]{4}} \implies \\
& \displaystyle 2x^2+2y^2-3 \sqrt[3]{2} x=0 \iff \left( x -\frac{3}{4}\sqrt[3]{2} \right)^2 + y^2 = \left( \frac{3}{4}\sqrt[3]{2} \right)^2
\end{array}$
Consequently, the locus is the circle centered in $\displaystyle \left( \frac{3}{4}\sqrt[3]{2},0 \right)$ and radius $\displaystyle \frac{3}{4}\sqrt[3]{2}$.
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