# F(x)=(x-1)(x+5) when f(1/x)

1. Oct 16, 2006

### CarlosRamos

I'm having difficulties in obtaining the book/calculator answer to the function
f(x) = (x-1)(x+5) when f(1/x).

The answer I get is: 1/(a^2) + 4a - 5
. or : (1 + 4a - 5a^2)/a^2
. or : [(1 + 4a)/a^2] - 5

I know that all these answers are possible answers, but I can't for the life of me figure out how the book got the answer (I've also checked with a graphing calculator, and recieved the same answers)

- Thanks :)

2. Oct 16, 2006

$$f(x) = (x-1)(x+5)$$
Then $$f(\frac{1}{a}) = (\frac{1}{a}-1)(\frac{1}{a} +5 )$$
Also it shoud be $$\frac{(1-a)(1+5a)}{a^{2}}$$