Length of Latus Rectum in Ellipses: A Geometric Proof

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In summary, a latus rectum in conics is a line segment that passes through the focus and is perpendicular to the major axis. Its length is equal to the distance from the focus to the directrix and can be found using the equation L = 2b²/a. It is significant in determining the eccentricity and key points of the conic, and can be negative when the focus and directrix are on opposite sides. The length of the latus rectum differs in different types of conics, being equal to the diameter in a parabola, half the length of the major axis in an ellipse, and equal to the distance between the two vertices in a hyperbola.
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DarkAnt
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"A line segment through a focus with endpoints on the ellipse and perpendicular to the major axis is a latus rectum of the ellipse. Therefore, an ellipse has two latus recta. Show that the length of each latus rectum is 2b^2/a."

I've been stuck on this for a little while now. Can anyone point me in the right direction?
 
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  • #2
Draw an ellipse, show the relationships of A and B, do a little trig...
 
  • #3
Substitute x = ae find y coordinate 2y will be the length of rectum
 
  • #4
Thank you :smile:
 

What is a latus rectum in conics?

A latus rectum in conics is a line segment that passes through the focus of a conic section and is perpendicular to the major axis. It is also equal in length to the distance from the focus to the directrix of the conic section.

How do you find the length of a latus rectum in conics?

The length of a latus rectum can be found by using the equation L = 2b²/a, where b is the length of the semi-minor axis and a is the length of the semi-major axis.

What is the significance of the latus rectum in conics?

The latus rectum is important in conics because it is used to determine the eccentricity of the conic section. It is also used to find the vertex, focus, directrix, and other key points of the conic.

Can you have a negative latus rectum in conics?

Yes, it is possible to have a negative latus rectum in conics. This occurs when the focus and directrix are on opposite sides of the conic. In this case, the latus rectum is still equal in length to the distance from the focus to the directrix, but it is measured in the opposite direction.

How does the latus rectum differ in different types of conics?

In a parabola, the latus rectum is equal to the diameter of the parabola. In an ellipse, the latus rectum is half the length of the major axis. In a hyperbola, the latus rectum is equal to the distance between the two vertices.

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