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27 December, 2020

focus of parabola

Latus rectum

Definition :-Latus rectum of a parabola is a line segment perpendicular to the axis of

the parabola, through the focus and whose end points lie on the parabola (Fig11.17).

To find the Length of the latus rectum of the parabola y2

 = 4ax (Fig 11.18).

By the definition of the parabola, AF = AC.

But AC = FM = 2a

Hence AF = 2a.

And since the parabola is symmetric with respect to x-axis AF = FB and so

AB = Length of the latus rectum = 4a.



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