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Thread: Parametrics question

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    Cadet elseany's Avatar
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    Parametrics question

    I cant seem to get this one out:

    P(2ap,ap2) & Q(2aq,aq2) lie on the parabola x2 = 4ay, where a > 0. The chord PQ passes through the focus.
    Show that the chord PQ has length A(p + 1/p)2

    thanks for your help

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    Re: Parametrics question

    Find the equation PQ, then find pq, by using the equation of the line and the point (0,a), and then use the distance formula.
    Is quite inactive.

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    Re: Parametrics question

    yeh i can find everything except it doesnt seem to work out when i do it with the distance formula.

    i've already shown that pq = -1

    but my working gets really messy with the distance formula and just doesnt solve :<

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    Re: Parametrics question

    yup lol, i remembered that.

    can somebody give the algebra a go?

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    jyu
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    Re: Parametrics question

    Quote Originally Posted by elseany
    yup lol, i remembered that.

    can somebody give the algebra a go?
    Length = sqrt[(x2 - x1)^2 + (y2 - y1)^2]
    =sqrt[(2ap -2aq)^2 + (ap^2 - aq^2)^2]
    =sqrt[4a^2(p - q)^2 + a^2(p^2 - q^2)^2]
    =sqrt[4a^2(p + 1/p)^2 + a^2(p^2 - 1/p^2)^2]
    =a sqrt[4(p + 1/p)^2 + (p^2 - 1/p^2)^2]
    =a sqrt[(p + 1/p)^2 (4 + (p - 1/p)^2)]
    =a sqrt[(p + 1/p)^2 (p + 1/p)^2]
    =a(p + 1/p)^2


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    Re: Parametrics question

    Or:

    let S be the focus (0,a), we know that SP=ap^2 + a = a(p^2 + 1)
    SQ = aq^2 + a = a(1/p^2 + 1) [think why?]
    so PQ = QS + SP = a (p^2 + 2 + 1/p^2) = a(p + 1/p)^2
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    Re: Parametrics question

    thanks jyu and affinity, but theres something i dont get;


    =a sqrt[(p + 1/p)^2 (4 + (p - 1/p)^2)]
    =a sqrt[(p + 1/p)^2 (p + 1/p)^2]

    how do you go from that first line to that second line?

  8. #8
    pLuvia
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    Re: Parametrics question

    Try expanding (4 + (p - 1/p)^2)] and it should turn out to be the same as (p + 1/p)^2]

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    Re: Parametrics question

    Quote Originally Posted by Affinity View Post
    Or:

    let S be the focus (0,a), we know that SP=ap^2 + a = a(p^2 + 1)
    SQ = aq^2 + a = a(1/p^2 + 1) [think why?]
    so PQ = QS + SP = a (p^2 + 2 + 1/p^2) = a(p + 1/p)^2
    Thanks this helps, (12 years later)!

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