Visualising maths

Parabolas - effect of parameter ‘a’

This section looks at the effect of changing the parameter $a$ in the Cartesian equation of the parabola $y^2 = 4 a x$. Changing the value of $a$ moves the position of the focus and the directrix, which in turn changes the curve. The smaller the value of $a$, the closer the focus and directrix are to the origin. Use the buttons to set the value of $a$ to different values, and see the effect on the curve. Read more →


We have previously seen how a parabola is defined in terms of parametric equations or alternatively in Cartesian form. An alternative way to define a parabola is as a locus of points. Focus and directrix The locus defining a parabola depends on a focus and a directrix. The focus is a point. For a standard parabola, the focus is located on the x axis a distance $a$ from the origin, that is at the point (a, 0). Read more →