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# Maths C3 question watch

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1. 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
2. (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 )
3. (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*
4. (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

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
5. (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)...
6. (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.
7. Alright, thanks everyone x

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