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Math Help - volum and radius

  1. #1
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    volum and radius

    Don't even know where to start..
    volume of a sphere has decreased by 30 percent. how many percentage has the radius decreased?
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  2. #2
    Super Member TheChaz's Avatar
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    Re: volum and radius

    When the radius of a sphere is multiplied by r,
    the area is multiplied by r^2
    the volume is multiplied by r^3.

    So when the volume is multiplied by .7 = r^3,
    r = ...
    (Then subtract this from "1", as I did to get .7, and convert to a percentage if needed)
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  3. #3
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    Re: volum and radius

    aha.. i realy don't get it :/
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  4. #4
    Super Member TheChaz's Avatar
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    Re: volum and radius

    Quote Originally Posted by tintin View Post
    aha.. i realy don't get it :/
    Well, we could break this into two parts.
    1) Concluding that r^3 = .7
    2) Solving for r in the equation in (1).

    Which don't you get?
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  5. #5
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    Re: volum and radius

    I don't know what you mean r^3 = .7.
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  6. #6
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    Re: volum and radius

    Tintin, did you at least google "volume of sphere"?
    Is this a problem given to you by your math teacher?
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  7. #7
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    Re: volum and radius

    No its in my math book. I managed doing it the other way around, like ''the radius of a sphere grows by 12,5 percent, how many percentage will the volume increase?''.
    Where i got ((1.125^3) -1)*100)=42%
    I just don't get it when it decreases..
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  8. #8
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    Re: volum and radius

    HINT:
    9^3 = 729 : 10 reduced by 10% to 9)
    10^3 = 1000
    11^3 = 1331 : 10 increased by 10% to 11)
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  9. #9
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    Re: volum and radius

    Hello tintin,
    V sphere =4/3 pi R^3
    V2 /V1 = .7/1= 4/3 pi R2^3/4/3pi R1^3
    V2/V1 = .7= R2^3/R1^3
    .7^1/3 = R2/R1 .887 R1 =R2
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