# Need help with something in a fuction

• Sep 5th 2007, 04:55 PM
Lithiux
Need help with something in a fuction
(Note: When I say delta, I mean the little triangle thing usually meaning change, I can't find the HTML code for it.)

Alright, I have a the function f(x) = x^2+3x-1

I then need to evaluate for f(x+deltax). I know that then should look something like x^2+3x-1 + deltax^2+3x-1.

Please verify that is essentially what should happen, and then help me understand what exactly I am supposed to do with the delta in front of the "x" value.
• Sep 5th 2007, 05:11 PM
Krizalid
Quote:

Originally Posted by Lithiux
Alright, I have a the function f(x) = x^2+3x-1

I then need to evaluate for f(x+deltax). I know that then should look something like x^2+3x-1 + deltax^2+3x-1.

$\displaystyle f(x+\Delta x)=(x+\Delta x)^2+3(x+\Delta x)-1$

Can you take it from there?
• Sep 5th 2007, 05:19 PM
Lithiux
Sorry, I r stoopid.

I'd gotten that far in my head as far as setting it up, but I'm not sure what to do with the delta. I'm not sure what is supposed to change.
• Sep 5th 2007, 05:32 PM
topsquark
Quote:

Originally Posted by Lithiux
Sorry, I r stoopid.

I'd gotten that far in my head as far as setting it up, but I'm not sure what to do with the delta. I'm not sure what is supposed to change.

You are not stupid. Stop that! You simply don't have a lot of experience.

Try thinking of $\displaystyle \Delta x$ as a single variable, not two symbols.

So for example
$\displaystyle (x + \Delta x)^2$
is the same kind of thing as
$\displaystyle (a + b)^2 = a^2 + 2ab + b^2$
where
$\displaystyle a = x \text{ and }b = \Delta x$

So
$\displaystyle (x + \Delta x)^2 = x^2 + 2x(\Delta x) + ( \Delta x)^2$

-Dan
• Sep 5th 2007, 05:43 PM
Lithiux
Ooooh, so to evaluate the delta, it would need to be an equation of some sort, and as such I just treat it as if it were an x or y or something like that. Cool, thanks.
• Sep 5th 2007, 06:17 PM
topsquark
Quote:

Originally Posted by Lithiux
Ooooh, so to evaluate the delta, it would need to be an equation of some sort, and as such I just treat it as if it were an x or y or something like that. Cool, thanks.

A $\displaystyle \Delta$ usually (not always) indicates the difference in some value. For example
$\displaystyle \Delta x = x_{final} - x_{initial}$
in Physics. But the point here is that $\displaystyle \Delta x$ is just a number.

-Dan
• Sep 5th 2007, 06:18 PM
Lithiux
Yeah, that was my problem. I was thinking of it in a Physics frame-of-mind.