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Maths C3 question watch

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    Hey all fellow mathmeticians out there
    I've been having trouble with this composite function for about 15 minutes now.
    The question is ;
    Functions f and g are defined by
    f(x) = 1/(x-1)(x+3), x > 1
    g(x) = 5/x
    Solve gf(x) = 25.
    The answer at the back of the book says 2 but i keep getting an insolveable quadratic, it's really bugging me now!
    Thanks for your help everyone xxx
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    (Original post by Remainresilient_)
    Hey all fellow mathmeticians out there
    I've been having trouble with this composite function for about 15 minutes now.
    The question is ;
    Functions f and g are defined by
    f(x) = 1/(x-1)(x+3), x > 1
    g(x) = 5/x
    Solve gf(x) = 25.
    The answer at the back of the book says 2 but i keep getting an insolveable quadratic, it's really bugging me now!
    Thanks for your help everyone xxx
    What's the insolveable quadratic? How did you get there? (Working please )
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    (Original post by SeanFM)
    What's the insolveable quadratic? How did you get there? (Working please )
    I got 5/(x-1)(x+3) = 25. after putting the function of f(x) into g(x). fast forward i keep getting the quadratic 25x^2 + 50x - 80. and this doesnt give me an answer of two *sigh*
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    (Original post by Remainresilient_)
    Hey all fellow mathmeticians out there
    I've been having trouble with this composite function for about 15 minutes now.
    The question is ;
    Functions f and g are defined by
    f(x) = 1/(x-1)(x+3), x > 1
    g(x) = 5/x
    Solve gf(x) = 25.
    The answer at the back of the book says 2 but i keep getting an insolveable quadratic, it's really bugging me now!
    Thanks for your help everyone xxx
    I got the answer,

    Heres a clue for what gf(x) equals hope it helps

    gf(x) = 5 / (1)/(x-1)(x+3)
    therefore gf(x) = 5(x-1)(x+3)

    EDIT: learn this http://i.prntscr.com/d516647df31a4b0...ca8edb142a.png
    • Very Important Poster
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    Very Important Poster
    (Original post by Remainresilient_)
    I got 5/(x-1)(x+3) = 25. after putting the function of f(x) into g(x). fast forward i keep getting the quadratic 25x^2 + 50x - 80. and this doesnt give me an answer of two *sigh*
    Remember that gf(x) means that you compute f(x) first, and then use that output and put it into g(x). I think you know/understand that but it has got a bit confusing with the fractions.

    f(x) gives you a fraction and putting that into g(x) gives you 5 / the fraction from f(x)...
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    (Original post by Remainresilient_)
    Hey all fellow mathmeticians out there
    I've been having trouble with this composite function for about 15 minutes now.
    The question is ;
    Functions f and g are defined by
    f(x) = 1/(x-1)(x+3), x > 1
    g(x) = 5/x
    Solve gf(x) = 25.
    The answer at the back of the book says 2 but i keep getting an insolveable quadratic, it's really bugging me now!
    Thanks for your help everyone xxx
    this is piss easy.
    • Thread Starter
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    Alright, thanks everyone x
 
 
 
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