The Parabola
To Draw a Parabola Envelope
Draw a fixed line AB and a fixed point S. Place a set square with right angle at Q on the line and with one side of the square resting on S. The other side of the set square will be tangent to the parabola.
Tangents to the Parabola
Place a point on the parabola. Place another point on the parabola so these two points and the focus are co-linear. The tangents at these two points intersect on the directirx at a right angle.
In other words, the tangents at the ends of a focal chord meet at right angles on the directirx.
The Diameter of a Parabola
A number of parallel chords are drawn. The midpoints of these chords lie on a line. Where this line intersects the parabola, the tangent at this point is parallel to the chords.