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Math Help - Related Rates

  1. #1
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    Related Rates

    I have trouble with related rate word problems; half of the time I don't know if the formula I'm using is the right one.

    For example, In one of the hw problems:

    A right circular cone has a height h = 8 ft, and the base radius r is increasing. Find the rate of change of its surface area S with respect to r when r= 6ft.

    The textbook provides a formula of the surface area as: (pi) * r * the square root of r^2 + h^2


    I know that I have to find dS/dr, but they give me two values, which confuses me

    [I'm still trying to learn how to right out problems and formulas using LaTex, I apologize.
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  2. #2
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    Re: Related Rates

    Find h in terms of r and substitute back into SA.
    Thanks from OmegaJudicator
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  3. #3
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    Re: Related Rates

    Quote Originally Posted by pickslides View Post
    Find h in terms of r and substitute back into SA.
    would I start with

    pi * r * rad(r^2 + h^2) and set it equal to 0, then square both sides? I know it sounds like a dumb question, I just feel nervous in every step I take in these kinds of problems.

    *What I did instead was find the surface area, then find h in terms of r. I got (60 - r^2) / r.
    Last edited by OmegaJudicator; March 28th 2012 at 09:01 PM.
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  4. #4
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    Re: Related Rates

    h=8 so S=pi.r.sqrt(r^2+64) Find dS/dR (you will need the product rule) and then substitute r=6 in your answer.
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