Trisectrix of Maclaurin

Cartesian equation: y^2(a+x) = x^2(3a-x)

Polar equation: r = 2a sin(3theta)/sin(2theta)

Click below to see one of the Associated curves.

Definitions of the Associated curves

Evolute       Involute 1       Involute 2

 Inverse curve wrt origin      Inverse wrt another circle

Pedal curve wrt origin      Pedal wrt another point

Negative pedal curve wrt origin      Negative pedal wrt another point

   Caustic wrt horizontal rays      Caustic curve wrt another point

This was first studied by Colin Maclaurin in 1742. Like so many curves it was studied to provide a solution to one of the ancient Greek problems, this one is in relation to the problem of trisecting an angle. The name trisectrix arises since it can be used to trisect angles.

The trisectrix of Maclaurin is an anallagmatic curve.

Another form of the equation is r = a sec(theta/3) where the origin is inside the loop and the crossing point is on the negative x-axis.

The tangents to the curve at the origin make angles of  + or - 60degrees with the x-axis.

The area of the loop is 3sqrt3a^2 and the distance from the origin to the point where the curve cuts the x-axis is 3a.

It is the pedal curve of the parabola where the pedal-point is taken as the reflection of the focus in the directrix.


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JOC/EFR February 1996