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#1 2005-08-12 22:52:49

KiWoonG
Guest

**_ Parametrics _**

Hi,,
i needed help for parametrics,,
here's a sample question,,,

A parabola has parametric equations x=8t and y=4t^2
Find the coordinates of its focus and equation of its directix.


can somebody help me on this??

#2 2005-08-13 00:14:52

mathsyperson
Moderator

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Re: **_ Parametrics _**

I'm not sure what you mean by focus and directix, but it's easy enough to convert it into a normal cartesian equation.
x=8t
t=x/8
y=4t
y=4(x/8)
y=4(x/64)
y=x/16

And from that, you should be able to get any information you want.


Why did the vector cross the road?
It wanted to be normal.

#3 2005-08-26 12:05:22

ajp3
Novice

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Re: **_ Parametrics _**

use the focus directrix form of the eqn of a parabola:

x = 4py (or y = 4px for the horizontal case); note that p = the distance from the vertex to the focus = the distance from the vertex to the directrix

now, mathsyperson has already done most of the work!

take x = 16y = 4(4y), so we see p = 4.

this means our parabola (with vertex (0,0) ) has focus (0, 4) and directrix y = -4

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