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Geometry Solids

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Geometry Solids

by ButtersBC » Tue Feb 24, 2009 8:58 am
So I just finished up with a Math Review book and are we really supposed to memorize the equations for the volume and surface areas of three dimensional solids? I couldn't do that in high school and have done a lot of drinking since then
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by marcusking » Tue Feb 24, 2009 9:08 am
I've seen some questions that have actually had the equations listed in them. But just to be safe I would memorize them. They aren't too difficult and make sense when you think about them, so memorization may not even be necessary.

Volume of a cylinder is Area of base * height. (find area of a circle and imagine lots of those stacked on top of one another)

Volume of a cube is measure of any side^3 (lenght*width*depth)

Volume of a sphere is (4/3)pi*r^3 (don't have an easy one for this)

Volume of a cone is (1/3)area of base * height. (a cone with the same height of a cylinder looks like it would take up about 1/3 of the volume)
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by ButtersBC » Tue Feb 24, 2009 9:15 am
It's when it gets into cones and what have you that I always misplace something...you can make an argument that a knowledge of algebra will always come in handy but geometry of that nature just makes me see red
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by marcusking » Tue Feb 24, 2009 9:40 am
ButtersBC wrote:It's when it gets into cones and what have you that I always misplace something...you can make an argument that a knowledge of algebra will always come in handy but geometry of that nature just makes me see red
I know the feeling we all have our weaknesses, but I'm sure you'll find over time things will get better. This board is full of bright people who really know there stuff and will take time to explain it to you. Welcome to beatthegmat.com and good luck on the test!
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by DanaJ » Tue Feb 24, 2009 9:45 am
You should never just try to memorize formulas: most of them make good sense (especially if it's not rocket science or smth).... Look at things this way:
There are three types of solids, IMHO:

I. You've got your "straight" solids: cube, cylinder, right prisms. For these babies the formula is (Area of base)*(height).

II. You've got the pyramid family; notable members are the pyramid and the cone. The general formula for this one is (Area of base)*(height)*1/3.

III. Last but not least, you've got the sphere, with its "weird" formula, IMO, although most math teachers will say it's part of the previous group. Here the formula is (4/3)*Pi*r^3, as previously mentioned. BUT: I can only remember one question from the GMAT prep tests involving the volume of a sphere, and the formula was provided (so you could have easily solved the problem even if you didn't know the formula).

IMO if you learn the group formulas you'll be fine on test day.
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