Equation of Ovals of Cassini/Cartesian Form

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Let $P_1$ and $P_2$ be points in the cartesian coordinate plane located at $\tuple {a, 0}$ and $\tuple {-a, 0}$ for some constant $a \ne 0$.

Let $b$ be a real constant.

The Cartesian equation:

$\paren {x^2 + y^2 + a^2}^2 - 4 a^2 x^2 = b^4$

describes the ovals of Cassini.



The ovals of Cassini are the loci of points $M$ in the plane such that:

$P_1 M \times P_2 M = b^2$

Let $b$ be chosen.

Let $P = \tuple {x, y}$ be an arbitrary point of $M$.

We have:

\(\displaystyle P_1 P\) \(=\) \(\displaystyle \sqrt {\paren {x - a}^2 + y^2}\) Distance Formula
\(\displaystyle P_2 P\) \(=\) \(\displaystyle \sqrt {\paren {x + a}^2 + y^2}\) Distance Formula
\(\displaystyle \leadsto \ \ \) \(\displaystyle \paren {P_1 P} \paren {P_2 P}\) \(=\) \(\displaystyle \sqrt {\paren {x + a}^2 + y^2} \sqrt {\paren {x - a}^2 + y^2}\) Definition of Ovals of Cassini
\(\displaystyle \) \(=\) \(\displaystyle b^2\)
\(\displaystyle \leadsto \ \ \) \(\displaystyle b^4\) \(=\) \(\displaystyle \paren {\paren {x + a}^2 + y^2} \paren {\paren {x - a}^2 + y^2}\) squaring both sides
\(\displaystyle \) \(=\) \(\displaystyle \paren {x^2 + y^2 + a^2 + 2 a x} \paren {x^2 + y^2 + a^2 - 2 a x}\) multiplying out and rearranging
\(\displaystyle \) \(=\) \(\displaystyle \paren {x^2 + y^2 + a^2}^2 - 4 a^2 x^2\) Difference of Two Squares

Hence the result.


Also see

Source of Name

This entry was named for Giovanni Domenico Cassini.