Defintion
- PF = Point P to focus F.
- PQ = Point P to directirx.
- PF = PQ then locus of P is a parabola.
- Equation of parabola is y + D/2 = x^2/(2*D).
- Where D is the distance from focus F to the directrix.
How to sketch using a ruler ?
- Sketch a fixed point F which is the focus.
- Sketch a fixed line which is the directrix.
- Skecth a line QR perpendicular to directrix where Q is on directrix.
- Join Q and F.
- Bisect QF and the bisector meet QR at P.
- Then P is point on the locus because PF = PQ.
- Use above procedures to find more points on locus.
- Joint all the points and we have a parabola.
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