# Thread: help finding a derivitive

1. ## help finding a derivitive

What are the steps to find the derivitive of a function, such as:
f(x) = x3 - 9x + 8

i know the formula is f(x)-f(x)/x-a
but idk how to use it

2. Originally Posted by kyleu03
What are the steps to find the derivitive of a function, such as:
f(x) = x3 - 9x + 8

i know the formula is f(x)-f(x)/x-a
but idk how to use it
No, you don't know "the formula". Please read that again and make sure you recognize the Order of Operations.

If you are supposed to be working on the formula, please review it, then implement it. The rest is algebra.

3. Originally Posted by TKHunny
No, you don't know "the formula". Please read that again and make sure you recognize the Order of Operations.

If you are supposed to be working on the formula, please review it, then implement it. The rest is algebra.

ok then what is the formula

4. Originally Posted by kyleu03
What are the steps to find the derivitive of a function, such as:
f(x) = x3 - 9x + 8

i know the formula is f(x)-f(x)/x-a
but idk how to use it
The correct formula is $\displaystyle \lim_{x\to a}\frac{f(x)- f(a)}{x-a}$. That is not at all what you wrote. In the first place, you don't have a limit. In the second, you have "- f(x)" instead of "f(a)". And finally, "f(x)- f(x)/x-a", without parentheses, means $\displaystyle f(x)- \frac{f(x)}{x}- a$"

In any case, you need to find
$\displaystyle \lim_{x\to a}\frac{x^3- 9x-8- (a^3- 9a- 8)}{x-a}= \lim_{x\to a}\frac{x^3-a^3- 9(x- a)}{x-a}$

If you use the fact that $\displaystyle x^3- a^3= (x-a)(x^2+ x+ 1)$ that will be fairly easy.