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    • Thread Starter

    OK, so I was working out what the formula for the surface area + volume of a torus would be and I got both right. Then I thought 'right, applying a similar principle to a sphere should work too'.

    So this is what happened:
    A sphere is basically a semicircle rotated around an axis. (works for a full circle too).
    It is rotated for the length of the full circle's circumference.

    And this is what I got..
    volume = 1/2.pi.r^2.2.pi.r
    = pi^2.r^3
    Using the semicircle-around-an-axis idea again;
    surface area = 2.pi.r.pi.r = 2.pi^2.r^2

    ... which is certainly not what it was supposed to be.

    So why doesn't this method work? What have I done wrong?
    This is probably a stupid question
    • Thread Starter

    Aha, never mind! I have to use the centroid of the semicircle rather than the radius... laziness does not pay of...
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Updated: November 3, 2013

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