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Math Help - Find the f'(x) at the given value of x.

  1. #1
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    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?
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by plstevens View Post
    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
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  3. #3
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    i got it, thanks so much
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    Quote Originally Posted by plstevens View Post
    i got it, thanks so much
    good!

    take care
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    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.
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by plstevens View Post
    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
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  7. #7
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    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
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    would it be x=0
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by plstevens View Post
    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
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  10. #10
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    i'm confused, so is x=0 wrong
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    Quote Originally Posted by plstevens View Post
    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
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  12. #12
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    ok thanks i see now
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