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Math Help - please help with this sphere problem..

  1. #1
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    please help with this sphere problem..

    One balloon had a volume of 108pi/3 cubic metres.
    a.what was the diameter of the balloon?
    b.The formula for the volume of a sphere is 4/3(pi)(r^2)
    what is the surface area of the balloon?
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  2. #2
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    The formula for volume is[Math] V = \frac{4}{3}\pi r^3[/tex]

    So you can just set the volume you have to the volume formula and solve for r, which is the radius. Then the diameter is just twice that.

    The formula for the area is 4 \pi r^2 so once you have the radius r, just plug it in in this.
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  3. #3
    Senior Member eumyang's Avatar
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    Assuming that the balloon is a perfect sphere...

    a) The volume of a sphere is
    V = \frac{4}{3}\pi r^3

    Plug in \frac{108\pi}{3} in for V:
    \frac{108\pi}{3} = \frac{4}{3}\pi r^3

    Multiply both sides by \frac{3}{4\pi}:
    \begin{aligned}<br />
\frac{108\pi}{3} \cdot \frac{3}{4\pi} &= \frac{4}{3}\pi r^3 \cdot \frac{3}{4\pi} \\<br />
27 &= r^3<br />
\end{aligned}

    Take the cube root of both sides:
    \begin{aligned}<br />
\sqrt[3]{27} &= \sqrt[3]{r^3} \\<br />
r &= 3<br />
\end{aligned}

    The diameter is double the radius, so the answer is 6 meters.

    EDIT: too slow! TT_TT
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  4. #4
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    yes i checked and this is correct (thank you)but how do i find the surface area?
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  5. #5
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    gabriel, for the surface area look at my post!
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  6. #6
    Senior Member eumyang's Avatar
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    Vlasev already told you.

    Quote Originally Posted by Vlasev View Post
    The formula for the area is 4 \pi r^2 so once you have the radius r, just plug it in in this.
    Since the radius is 3 meters...
    \begin{aligned}<br />
SA &= 4\pi r^2 \\<br />
&= 4\pi \cdot 3^2 \\<br />
&= 36\pi<br />
\end{aligned}
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