# Homework Help: Need Help With Derivative Function

1. Feb 12, 2008

### kwikness

1. The problem statement, all variables and given/known data
Find the Derivative Function of (4x - x$$^{2}$$)

3. The attempt at a solution

using formula:

$$\frac{dy}{dx} = \frac{f(x + \Delta x) - f(x)}{\Delta x}$$

$$\frac{4(x + \Delta x) - (-x^{2})}{\Delta x}$$

$$\frac{4x + 4(\Delta x) + x^{2}}{\Delta x}$$

Not sure where to go from here..

Last edited: Feb 12, 2008
2. Feb 12, 2008

### EnumaElish

Let d = $\Delta$x.

f(x+d) - f(x) = 4(x+d)-(x+d)^2 - 4x + x^2

which simplifies to (4 - d - 2x)d (you should derive this). The rest should be easy.

3. Feb 12, 2008

### CompuChip

First of all, you mean
$$\frac{dy}{dx} = \lim_{\Delta x \to 0} \frac{f(x + \Delta x) - f(x)}{\Delta x}$$
as what you gave is just the difference quotient
$$\frac{\Delta y}{\Delta x} = \frac{f(x + \Delta x) - f(x)}{\Delta x}$$

Not sure how you got there in the first place. If I plug $f(x) = 4x - x^2$ into the formula you gave, I get
$$\frac{ [ 4(x + \Delta x) - (x + \Delta x)^2 ] - [4 x - x^2 ] }{ \Delta x } = \frac{ 4 x + 4 \Delta x - x^2 - 2 x \Delta x - (\Delta x)^2 - 4 x + x^2 }{ \Delta x}$$
which has some terms you don't have. Now try again.

4. Feb 12, 2008

### HallsofIvy

That's because you haven't finished the algebra! You have 4x and -4x in the numerator! You have -x2 and x2 in the numerator!

5. Feb 12, 2008

### CompuChip

Exactly, and the -4x and -x2 just happen to be some of the terms kwikness is missing
But I'm leaving him some work.

6. Feb 12, 2008

### kwikness

Thanks, when I wrote it down I was missing a part of the equation. Gahhh! I always make stupid mistakes like that.

7. Feb 12, 2008

### Hydrargyrum

You could just have used the Power Rule, but I guess you haven't learned it yet.