Straight Line

Cartesian equation: y = mx + c or x = t, y = mt +c

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

The straight line must be one of the earliest curves studied, but Euclid in his Elements although he devotes much study to the straight line, does not consider it a curve.

In fact nobody attempted a general definition of a curve until Jordan in his Cours d'Analyse in 1893.

The inverse of a straight line is a circle if the centre of inversion is not on the line.

The negative pedal of the straight line is a parabola if the pedal point is not on the line.

Since normals to a straight line never intersect and tangents coincide with the curve, evolutes, involutes and pedal curves are not too interesting.


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JOC/EFR February 1996