# Find the surface area of an open-topped box

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• November 15th 2011, 01:19 PM
Dragon08
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Find the surface area of an open-topped box
Attachment 22758

Find the surface area of this open-topped lid.

I know the formula for the surface area of a cylinder is pi(r^2)h, but I don't know how to calculate the inside of the lid.
• November 15th 2011, 01:43 PM
takatok
Re: Find the surface area of an open-topped box
Actually
$\pi r^2 h$ is the volume of a cylinder.

$2 \pi r^2+ 2 \pi r h$ is the Surface Area.

If you think about it you'll see why. You need the area of the circle on top, and bottom which is $\pi r^2$ * 2.

Now the circumference of those circles is $2 \pi r$ and we need to add all of the circumferences from top to bottom. So we multiply by h. This gives us the edge all the way around connecting the top to the bottom.Now this gives the total Surface Area.

Thinking about it I forgot to add. If you cut a hole in the cylinder you have to consider adding up the circumferences of the inside you cut out since they are not on the surface.

I think your drawing is like a mason jar lid. A cylinder with a hole all the way through from top to bottom. So what pieces from the total surface area are missing? Figure that out and subtract it from the above formula.

N.B: Sorry forgot to add. Since we are cutting a hole through the middle of this thing. We need to find the circumferences of the inside ring and multiply by h. Then ADD that to the total, since that is now part of the surface and wasn't before.
• November 15th 2011, 01:50 PM
Soroban
Re: Find the surface area of an open-topped box
Hello, Dragon08!

Could you state the original probem?
The given description makes no sense.

Quote:

Find the surface area of this open-topped lid. . What "lid"?

I know the formula for the surface area of a cylinder is pi(r^2)h . . . . no
but I don't know how to calculate the inside of the lid.

Your title refers to an open-topped box.

Then you refer to the volume of a cylinder.

I don't see any "lid" in this problem.
Your sketch is of a thick-walled cylinder.
. . Its height is 500 cm.
. . Its outer radius is 50 cm.
. . Its inner radius is 49 cm.

If you intend to paint the visible surface of the cylinder,
. . we want the inside surface area, the outside surface area,
. . and the area of the two "rings" at the top and bottom.

*
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