Tangent to a Cycloid
The cycloid is the locus of a point on a circle rolling along a straight line. This Demonstration illustrates a theorem proved by Descartes: the tangent to the cycloid at a point is perpendicular to , where is the point of contact of the circle and the line.
 University of St Andrews Scotland, School of Mathematics and Statistics. The MacTutor History of Mathematics archive. "Cycloid." (Nov 29, 2012) www-history.mcs.st-and.ac.uk/Curves/Cycloid.html.
"Tangent to a Cycloid"
Wolfram Demonstrations Project
Published: November 29 2012