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Math Help - Another question

  1. #1
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    Another question

    If f(x) =  \frac {1} {\sqrt {x} } and g(x) = 1 -  x^2,find f(g(x)).
    Last edited by Samantha; July 3rd 2007 at 05:14 AM.
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  2. #2
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    Hello, Samantha!

    Exactly where is your difficulty?


    If f(x) = \frac {1} {\sqrt{x}} and g(x) = 1 - x^2, find f(g(x)).
    We have: . f(g(x)) \;=\;f\left(1-x^2\right) \;=\;\frac{1}{\sqrt{1-x^2}}

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  3. #3
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    Actually, I was just making sure that I'm right...


    I have a problem with word problems

    The volume of a right circular cone of radius r and height r is given by V=  \frac {\pi} {3} r^3. How fast is the volume changing with respect to changes in r when the radius is equal to 2 feet?.
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  4. #4
    Bar0n janvdl's Avatar
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    Quote Originally Posted by Samantha View Post
    Actually, I was just making sure that I'm right...


    I have a problem with word problems

    The volume of a right circular cone of radius r and height r is given by V=  \frac {\pi} {3} r^3. How fast is the volume changing with respect to changes in r when the radius is equal to 2 feet?.
    Oh, you have a different formula.

     \frac {\pi} {3} r^3

    now  \frac {\pi} {3} is a constant.

    So  \frac {dV} {dr} = (3)( \frac {\pi} {3} r^2)

      = \pi r^2

    Set r = 2

      = \pi 2^2

      = 4 \pi
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  5. #5
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     <br />
(\frac (\pi) {3}) = V = \frac (\pi) {3} * r * r^2<br /> <br />

    r^2 = 4

    v = \frac (\pi) {3} * 2 * (r)^2 = \frac (\pi) {3}*1(2^2)= 4 \frac (\pi) {3}
    [/tex]

    Am I right?
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  6. #6
    Bar0n janvdl's Avatar
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    Quote Originally Posted by Samantha View Post
     <br />
(\frac (\pi) {3}) = V = \frac (\pi) {3} * r * r^2<br /> <br />
r^2 = 4<br /> <br />
v = \frac (\pi) {3} * 2 * (r)^2 = \frac (\pi) {3}*1(2^2)= 4 \frac (\pi) {3}<br />

    Am I right?
    Hmm, seems you are having problems with latex.
    Just check my latest reply. I think it's right.
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  7. #7
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    Some how I got 4  \frac \pi {3}
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  8. #8
    Bar0n janvdl's Avatar
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    Quote Originally Posted by Samantha View Post
    Some how I got 4  \frac \pi {3}
    I cant see how you got it.
    Im struggling to read your reply.
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  9. #9
    Bar0n janvdl's Avatar
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    Quote Originally Posted by Samantha View Post
     <br />
(\frac (\pi) {3}) = V = \frac (\pi) {3} * r * r^2<br /> <br />

    r^2 = 4

    v = \frac (\pi) {3} * 2 * (r)^2 = \frac (\pi) {3}*1(2^2)= 4 \frac (\pi) {3}
    [/tex]

    Am I right?
    No, why did you say  r * r^2 ????

    And why is it  \frac{3}{ \pi } in your calculations there?
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  10. #10
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    <br />
(\frac \pi {3})<br />
= \mathbb{V}_{1} = \frac \pi {3} * r^2
     <br />
2^2 = 4<br />


    \mathbb{V}_{2} = \frac \pi {3} * r  ({r}_{2})^2 = \frac \pi {3} * 1 (2^2) = 4 \frac \pi {3}
    Last edited by Samantha; July 3rd 2007 at 06:30 AM.
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  11. #11
    Bar0n janvdl's Avatar
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    Samantha, i really think your answer is wrong...

    I'll ask Jhevon or someone to check mine though.
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  12. #12
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    Can u look at my answer again? It's on the first page =)
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  13. #13
    Bar0n janvdl's Avatar
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    Quote Originally Posted by Samantha View Post
    Can u look at my answer again? It's on the first page =)
    I checked it again, and no, i dont agree with it, sorry.
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  14. #14
    Bar0n janvdl's Avatar
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    Quote Originally Posted by Samantha View Post
    <br />
(\frac \pi {3})<br />
= \mathbb{V}_{1} = \frac \pi {3} * r^2
     <br />
2^2 = 4<br />


    \mathbb{V}_{2} = \frac \pi {3} * r  ({r}_{2})^2 = \frac \pi {3} * 1 (2^2) = 4 \frac \pi {3}
    Explain \frac \pi {3} * r^2

    Why did you use r^2 ????
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  15. #15
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by Samantha View Post
    Actually, I was just making sure that I'm right...


    I have a problem with word problems

    The volume of a right circular cone of radius r and height r is given by V=  \frac {\pi} {3} r^3. How fast is the volume changing with respect to changes in r when the radius is equal to 2 feet?.
    Quote Originally Posted by janvdl View Post
    Explain \frac \pi {3} * r^2

    Why did you use r^2 ????
    I see what's the point of confusion here.

    The volume of a right-cylinder is given by V = \frac { \pi}{3} r^2 h, however, in this particular cone, h = r. This is why Samantha made a distinction between the r's and used r \cdot r^2. realizing that the first r is for the height, she decided not to differentiate that one. However, this is incorrect. Janvdl is right. if h = r it is perfectly valid to treat the h as r in every way. in fact, this is a prevalent related rates technique. a lot of questions like this require you to solve for one variable in terms of another so you can differentiate a single variable function, we do not make distinctions between the same variable despite the fact it came from two different quantities.

    even if your method was right Samantha, you would still get it wrong, why did you plug in 1 for the r that represents the height? if h is equal to r, then when r = 2, the height would be the same. and looking at it again, it seems you didn't differentiate at all. i'm sorry but what you did is confusing (i am looking at your last attempt at the question). can you explain exactly what you were doing so that we can clear up any misconceptions you have?
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