Q.If the parabola $y^2 = 4ax$ passes through the point $(3, 2)$, then the length of its latus rectum is
(A) $\dfrac{2}{3}$
(B) $\dfrac{4}{3}$
(C) $\dfrac{1}{3}$
(D) $4$

Standard Equations of a Parabola

A parabola is the set of all points in a plane that are equidistant from a fixed point (the focus) and a fixed line (the directrix). To turn this definition into a clean equation, NCERT places the parabola in the simplest position: vertex at the origin with its axis along a coordinate axis. The equations you get are called the standard equations.

The four standard forms

Depending on which way the parabola opens, there are four standard equations. In each, $a > 0$.

For all four the vertex is at the origin $(0,0)$ and the axis of the parabola is a coordinate axis.

Where $y^2 = 4ax$ comes from

Take the focus at $F(a, 0)$ and the directrix as the line $x = -a$. For a point $P(x, y)$ on the parabola, its distance to the focus equals its distance to the directrix:

Squaring both sides:

and expanding gives $y^2 = 4ax$. The other three forms follow by turning the focus in a different direction.

Latus rectum

The latus rectum is the chord through the focus, perpendicular to the axis, with both ends on the parabola. For every standard parabola its length is $4a$ — the very same $4a$ that appears in the equation, which makes it quick to read off.

Worked example

For the parabola $y^2 = 12x$, compare with $y^2 = 4ax$: here $4a = 12$, so $a = 3$.

• Vertex: $(0, 0)$
• Focus: $(a, 0) = (3, 0)$
• Directrix: $x = -3$ …