# Thread: Find the f'(x) at the given value of x.

1. ## Find the f'(x) at the given value of x.

f(x)=square root of x; Find F'(9).

Here what i did. I simple plugged 9 in for x, and i got 3, but thats obviously wrong. The reason i tried this is because at first i tried to find using the derivative formula but i got 0/0 which is wrong also. where did i go wrong?

2. Originally Posted by plstevens
f(x)=square root of x; Find F'(9).

Here what i did. I simple plugged 9 in for x, and i got 3, but thats obviously wrong. The reason i tried this is because at first i tried to find using the derivative formula but i got 0/0 which is wrong also. where did i go wrong?
you need to find the derivative

you have to use the formula?

there are two formulas you can use. the more popular one seems to be

$f'(x) = \lim_{h \to 0} \frac {f(x + h) - f(x)}h$

hint: 0/0 is not the answer you should get try rationalizing the numerator to simplify

3. i got it, thanks so much

4. Originally Posted by plstevens
i got it, thanks so much
good!

take care

5. should the answer be 1/6 after i rationalize? thats what i'm getting now. and also how do u put in formulas cause id on't know how to do that, so i always find my self typing in the form of words.

6. Originally Posted by plstevens
should the answer be 1/6 after i rationalize? thats what i'm getting now.
yes, that is correct

and also how do u put in formulas cause id on't know how to do that, so i always find my self typing in the form of words.
to get the pretty math symbols, we use LaTeX. see the tutorial here

7. ## Find the x-values where the function does not have a derivative.

Its a graph. The graph is the shape of a "v" opening down. The point of the, where the two lines meet to form the v, are on the y axis at 1, the line on the left side runs through the point -1, and the line on the right side runs through the point 1.
Now, i couldn't figure this out for the love of God, i have two problems like this but if you could help me with this one, i'm sure i'll be able to do the other one on my own

8. would it be x=0

9. Originally Posted by plstevens
Its a graph. The graph is the shape of a "v" opening down. The point of the, where the two lines meet to form the v, are on the y axis at 1, the line on the left side runs through the point -1, and the line on the right side runs through the point 1.
Now, i couldn't figure this out for the love of God, i have two problems like this but if you could help me with this one, i'm sure i'll be able to do the other one on my own
if the limit i posted above does not exist for some point, then the derivative does not exist at that point.

also, as far as graphs are concerned, we cannot differentiate at a point where there is a sharp corner or cusp or where the function is undefined

10. i'm confused, so is x=0 wrong

11. Originally Posted by plstevens
i'm confused, so is x=0 wrong
i don't know, you never told me for what x-value the point of the "V" occurs. that's where it is not differentiable, as i said. the point of the "V" is a sharp turn. if it occurs at x = 0, then that's it

12. ok thanks i see now