What is the directrix of a parabola?
The locus of the point that is equidistant from a point and a straight line is called a parabola. The point is called the focus of the parabola and the straight line is called the directrix.
If (x, y) is a point on a parabola having
If the focus is (a, 0) and the directrix is x + a = 0. this becomes