Editing Parabola (section)

== Pin and string construction ==
[[File:Parabola-pin-string.svg|thumb|Parabola: pin string construction]]
The definition of a parabola by its focus and directrix can be used for drawing it with help of pins and strings:<ref>Frans van Schooten: ''Mathematische Oeffeningen'', Leyden, 1659, p. 334.</ref>
# Choose the ''focus'' <math>F</math> and the ''directrix'' <math>l</math> of the parabola.
# Take a triangle of a ''set square'' and prepare a ''string'' with length <math>|AB|</math> (see diagram).
# Pin one end of the string at point <math>A</math> of the triangle and the other one to the focus <math>F</math>.
# Position the triangle such that the second edge of the right angle is free to ''slide'' along the directrix.
# Take a ''pen'' and hold the string tight to the triangle.
# While moving the triangle along the directrix, the pen ''draws'' an arc of a parabola, because of <math>|PF| = |PB|</math> (see definition of a parabola).