# Domain of function

• Jun 6th 2006, 02:35 PM
starbrite
Algebra question
1/ x^2-10x - 8
I know I have to solve for zero and factor it but when I do it just comes out 1, -5 but the awnser is -1 ,5 ... I just dont understand why ? Sorry math is the subject I struggle with most .Thank you
• Jun 6th 2006, 02:45 PM
ThePerfectHacker
Quote:

Originally Posted by starbrite
1/ x^2-10x - 8
I know I have to solve for zero and factor it but when I do it just comes out 1, -5 but the awnser is -1 ,5 ... I just dont understand why ? Sorry math is the subject I struggle with most .Thank you

Where is the equation!

$\displaystyle \frac{1}{x^2-10x-8}=??$
• Jun 6th 2006, 02:48 PM
starbrite
oh sorry its f(x)
I have to find the domain of the function . Thanks
• Jun 6th 2006, 02:55 PM
ThePerfectHacker
Quote:

Originally Posted by starbrite
oh sorry its f(x)
I have to find the domain of the function . Thanks

It math course they teach you whenever they write a function they always assume all possible values that 'x' can take since you have a fraction,
$\displaystyle \frac{1}{x^2-10x-8}$
It can be any value EXCEPT when the denominator is zero.
Thus,
$\displaystyle x^2-10x-8\not =0$
$\displaystyle x=\frac{10\pm \sqrt{100+64}}{2\cdot 1}$
$\displaystyle x=\frac{10\pm 2\sqrt{41}}{2}$
$\displaystyle x=5\pm \sqrt{41}$
$\displaystyle \sqrt{100+64}=\sqrt{164}=\sqrt{4\cdot 41}=2\sqrt{41}$