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Math Help - Cube

  1. #1
    Newbie
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    Mar 2010
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    Cube

    Don't know if this is the right section. Sincerely hope so.

    I'm having trouble with this problem (I'll try to translate it right...).

    The edge of a cube measures x/2 - 1, in cm. Determine the rate of change in volume for x = 10.

    I tried to find the derivative of [(x/2 - 1)^3] and then the result of x = 10, but I didn't get 24 cm3 per 1 cm, which is the right answer. I'ld like to know what I'm doing wrong.
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  2. #2
    Junior Member
    Joined
    Apr 2010
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    You probably got the derivative wrong.. it's

    \frac{d}{dx}\left(\frac{x}{2}-1\right)^3 = \frac{3}{2}\left(\frac{x}{2}-1\right)^2

    Filling in x=10 yields the desired result.
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  3. #3
    Newbie
    Joined
    Mar 2010
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    I had it this way.

    \frac{d}{dx}\left(\frac{x}{2}-1\right)^3 ={3}\left(\frac{x}{2}-1\right)^2

    I see what I did wrong. Thank you, brouwer.
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