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# Conics watch

1. Stuck on this one:

a parabola has a cartesian equation y^2=24x

a tangent y=mx+c is called P. show that mc is a constant, finding its value.

I haven't a clue, i've worked out a = 6. (in the 4ax thing) but that's all i've got.

Any suggestions?
2. (Original post by Discutimos)
Stuck on this one:

a parabola has a cartesian equation y^2=24x

a tangent y=mx+c is called P. show that mc is a constant, finding its value.

I haven't a clue, i've worked out a = 6. (in the 4ax thing) but that's all i've got.

Any suggestions?
this is a bit tough for the current FP1 in EDEX

is this homework?
3. (Original post by TeeEm)
this is a bit tough for the current FP1 in EDEX

is this homework?
This is AQA and it is a past paper question from 2003, according to the textbook.
4. (Original post by Discutimos)
This is AQA and it is a past paper question from 2003, according to the textbook.
I will run you through it

solve simultaneously
get expression in m, c and x
arrange as a quadratic in x
discriminant =0 (Tangent)
simplifies very nicely
this would give mc = .... number, i.e a constant
5. (Original post by TeeEm)
I will run you through it

solve simultaneously
get expression in m, c and x
arrange as a quadratic in x
discriminant =0 (Tangent)
simplifies very nicely
this would give mc = .... number, i.e a constant
Tried the first bit XD

24x=m^2x^2 + 2mxc + c^2

Do you compare coefficents so 2mc=24?
6. (Original post by Discutimos)
Tried the first bit XD

24x=m^2x^2 + 2mxc + c^2

Do you compare coefficents so 2mc=24?
arrange as a quadratic in x
7. (Original post by TeeEm)
arrange as a quadratic in x
m^2x^2 + 2mcx - 24x + c^2 = 0
8. (Original post by Discutimos)
m^2x^2 + 2mcx - 24x + c^2 = 0
write the coefficient of x sensibly before you consider the discrminant
9. (Original post by TeeEm)
write the coefficient of x sensibly before you consider the discrminant

well, it's either going to be 2mc-24 or mc-12
10. (Original post by Discutimos)
well, it's either going to be 2mc-24 or mc-12
The first one
(the second one is not correct)
11. (Original post by TeeEm)
The first one
(the second one is not correct)

Ok good cause i thought it was the first then you said sensibly and i got worried XD.

I got it now, i see the discriminant simplifies to 576 = 96mc, 576/96=mc=6

I doubted what i was doing too early on, all that csqred and msqred stuff threw me off completely, thanks for your help!
12. (Original post by Discutimos)
Ok good cause i thought it was the first then you said sensibly and i got worried XD.

I got it now, i see the discriminant simplifies to 576 = 96mc, 576/96=mc=6

I doubted what i was doing too early on, all that csqred and msqred stuff threw me off completely, thanks for your help!
all correct
this is a standard result for parabolas...
13. (Original post by TeeEm)
this is a standard result for parabolas...
What do you mean?
14. (Original post by Discutimos)
What do you mean?
always true for a tangent on a parabola of the form y2 =4ax
15. (Original post by TeeEm)
always true for a tangent on a parabola of the form y2 =4ax
What as in this is a fullproof method? im not sure what you are saying is always true
16. (Original post by Discutimos)
What as in this is a fullproof method? im not sure what you are saying is always true
do not worry ...

It is of no importance to your question
17. (Original post by TeeEm)
do not worry ...

It is of no importance to your question
Well alrighty then XD, thanks again!
18. (Original post by Discutimos)
Well alrighty then XD, thanks again!
no worries

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