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    I tried to do a substitution with 2 equations but i got a really horrible quadratic with massive numbers and of course it didn't factorise but when i came to use the formula on it, it turned out the formula didn't work anyway.


    How am i supposed to approach this question? The second part finding K and r
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    (Original post by will'o'wisp2)
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    I tried to do a substitution with 2 equations but i got a really horrible quadratic with massive numbers and of course it didn't factorise but when i came to use the formula on it, it turned out the formula didn't work anyway.


    How am i supposed to approach this question? The second part finding K and r
    Given the answers are to 1 dec.pl, I wouldn't expect the quadratic to factorise.

    I think it's just a case of ploughing through with the formula - which did work when I tried. I wasn't being careful, and there may be an error. I ended up with

    15r^2-110r+104=0

    Arithmetic slip somewhere?
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    (Original post by ghostwalker)
    Given the answers are to 1 dec.pl, I wouldn't expect the quadratic to factorise.

    I think it's just a case of ploughing through with the formula - which did work when I tried. I wasn't being careful, and there may be an error. I ended up with

    15r^2-110r+104=0

    Arithmetic slip somewhere?
    hmm, let me try again i'll report back asap
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    (Original post by ghostwalker)
    Given the answers are to 1 dec.pl, I wouldn't expect the quadratic to factorise.

    I think it's just a case of ploughing through with the formula - which did work when I tried. I wasn't being careful, and there may be an error. I ended up with

    15r^2-110r+104=0

    Arithmetic slip somewhere?
    man i get 24r^2  -310 +351

    but nvm it's ok forget this one for now, i'll let it rest
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    (Original post by will'o'wisp2)
    man i get 24r^2  -310 +351

    but nvm it's ok forget this one for now, i'll let it rest
    Just worked out a different route, avoiding the quadratic.

    In order to avoid the subscripts, I'll use T for P_{n+1} and S for P_n

    Our formula then becomes:

    \displaystyle T=\frac{KrS}{K+(r-1)S}

    Which after multplying up, and grouping the K terms together, gives:

    \displaystyle (T-rS)K = (1-r)ST

    Plug in the two sets of values for S and T to get two equations, in K and r.

    Then divide one equation by the other, eliminating K and the 1-r, and after multiplying up, you're left with a linear equation in r.
 
 
 

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