# Tschirnhausen cubic

In geometry, the Tschirnhausen cubic, or Tschirnhaus' cubic is a plane curve defined, in its left-opening form, by the polar equation

$r=a\sec ^{3}(\theta /3).$ ## History

The curve was studied by von Tschirnhaus, de L'Hôpital, and Catalan. It was given the name Tschirnhausen cubic in a 1900 paper by R C Archibald, though it is sometimes known as de L'Hôpital's cubic or the trisectrix of Catalan.

## Other equations

Put $t=\tan(\theta /3)$ . Then applying triple-angle formulas gives

$x=a\cos \theta \sec ^{3}{\frac {\theta }{3}}=a(\cos ^{3}{\frac {\theta }{3}}-3\cos {\frac {\theta }{3}}\sin ^{2}{\frac {\theta }{3}})\sec ^{3}{\frac {\theta }{3}}=a\left(1-3\tan ^{2}{\frac {\theta }{3}}\right)$ $=a(1-3t^{2})$ $y=a\sin \theta \sec ^{3}{\frac {\theta }{3}}=a\left(3\cos ^{2}{\frac {\theta }{3}}\sin {\frac {\theta }{3}}-\sin ^{3}{\frac {\theta }{3}}\right)\sec ^{3}{\frac {\theta }{3}}=a\left(3\tan {\frac {\theta }{3}}-\tan ^{3}{\frac {\theta }{3}}\right)$ $=at(3-t^{2})$ giving a parametric form for the curve. The parameter t can be eliminated easily giving the Cartesian equation

$27ay^{2}=(a-x)(8a+x)^{2}$ .

If the curve is translated horizontally by 8a and the signs of the variables are changed, the equations of the resulting right-opening curve are

$x=3a(3-t^{2})$ $y=at(3-t^{2})$ and in Cartesian coordinates

$x^{3}=9a\left(x^{2}-3y^{2}\right)$ .

This gives the alternative polar form

$r=9a\left(\sec \theta -3\sec \theta \tan ^{2}\theta \right)$ .