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Why is the equation of the tangent passing through X and P (from part a) not the same as the line passing through X and P
3F Question 10d equation of XP
https://644625398389466aee00633223056f55519d5fbc.googledrive.com/host/0B1ZiqBksUHNYQWY1UWtNa0NRNU0/CH3.pdf
(edited 7 years ago)
Original post by Acrux
Why is the equation of the tangent passing through X and P (from part a) not the same as the line passing through X and P
3F Question 10d equation of XP
https://644625398389466aee00633223056f55519d5fbc.googledrive.com/host/0B1ZiqBksUHNYQWY1UWtNa0NRNU0/CH3.pdf


They are the same - just using different parameters.

We have a=ct, so t=a/c. Substitute that into the first equation for the tangent, and you end up with the second equation.

Method used in the markscheme is a bit round the houses, IMO.
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Acrux
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7
Original post by ghostwalker
They are the same - just using different parameters.

We have a=ct, so t=a/c. Substitute that into the first equation for the tangent, and you end up with the second equation.

Method used in the markscheme is a bit round the houses, IMO.


wait so what do i do?
put a/c in tangent x+t^2y=2ct
Original post by Acrux
wait so what do i do?
put a/c in tangent x+t^2y=2ct


Yep. Just substitute a/c for t in that equation, and you'll get their second equation.
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Acrux
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Original post by ghostwalker
Yep. Just substitute a/c for t in that equation, and you'll get their second equation.


I got ya^2=2a-c^2x
rearragned to get

Actually y=-c^2x/a^2 + 2c^2/a
is this correct @ghostwalker
(edited 7 years ago)
Original post by Acrux
I got ya^2=2a-c^2x
rearragned to get

Actually y=-c^2x/a^2 + 2c^2/a
is this correct @ghostwalker


Take out the factor as per the markscheme, and you can see for yourself.

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