# Math Help - Calc 1 question

1. ## Calc 1 question

find the derivitive f(x) = -4/x^2 use the limit process

Using fx=(x+Dx)-fx/Dx

I get -4/(x+Dx)^2/x^2 + 4/x^2 and get lost in the math - am I on the right track?

2. Hello,

Originally Posted by weezie23
find the derivitive f(x) = -4/x^2 use the limit process

Using fx=(x+Dx)-fx/Dx

I get -4/(x+Dx)^2/x^2 + 4/x^2 and get lost in the math - am I on the right track?

$f'(x)=\frac{f(x+Dx)-f(x)}{Dx}$

$f({\color{red}x+Dx})=\frac{-4}{({\color{red}x+Dx})^2}$

---> $f'(x)=\frac{\frac{-4}{(x+Dx)^2}+\frac{4}{x^2}}{Dx}$

Did you catch your mistake ? Now, try to simplify (show your work, it'd be better)

3. It's time to get UNlost. You just have to sort through it.

These are NOT the same:

x+3/5 and (x+3)/5

2) Don't write things that make no sense.

You have: fx=(x+Dx)-fx/Dx

Notice how "fx" is on both sides. I'm sure you didn't mean that. Plus, there's and 'f' missing on the right -- f(x+Dx)?

4. As far as I get is 8x^2+8xDx+4Dx^2/(x+Dx)-x^2

Originally Posted by Moo
Hello,

$f'(x)=\frac{f(x+Dx)-f(x)}{Dx}$

$f({\color{red}x+Dx})=\frac{-4}{({\color{red}x+Dx})^2}$

---> $f'(x)=\frac{\frac{-4}{(x+Dx)^2}+\frac{4}{x^2}}{Dx}$

Did you catch your mistake ? Now, try to simplify (show your work, it'd be better)

5. You didn't listen to me about the notation, did you?

If you are VERY CAREFUL, you will see it. If you are sloppy, it will work only by luck.