# math

posted by on .

2 rectangular prisms are proportional. the edges of the smaller prism are half as long as the edges of the larger prism. The volume of the larger prism is 1,024 cubic feet. the volume of the smaller prism is what?
thank you.

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volume proportional to scale ratio cubed
1/2*1/2*1/2 = 1/8
so
1024/8 = 128

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thank you Damon, but how did you get that equation?

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Well, you see I am a Naval Architect (ship designer)
We test ships in model basins
say the ship is k times as long as the model
then it is k times as deep
and it is k times as wide
so its volume is k * k * k = k^3 times the volume of the model :)
If you imagine the model to be made up of tiny cubes
then the ship would have to have k times as many rows along the length
and k times as many columns in width
and k times as many high
so it would need k^3 times the original number of tiny cubes to fill it .

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i know how to get volume,but i need to know how to find the volume of a prism. this prism is proportional to a smaller prism. but the edges of the smaller prism is half as long as the edges of the larger prism. the volume of the larger prism is 1,024 cubic feet. then the volume of the smaller prism is what? i still do not get it. by the way, that's interestinng that you design ships.

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are you still there?

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if the small prism is proportional to the large prism
Then the height of the small prism is proportional to the height of the large prism
H = k h
and the length is also proportional
L = k l
and the width is proportional
W = k w
so
L W H = k l * k w * k h = k^3 lwh

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That is why I answered your cylinder problem further down the same way. You do not have to know pi.

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oh, but if the volume of the large prism is 1024, and the edges of the smaller prism is half the length of the larger prism, then what does k mean in your explanation?

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every edge and other length measurement on the little one is 1/2 that on the big one
so k = 1/2

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It does not matter the shape as long as the two objects have the same shape.
The ratio of their volumes is the scale ratio cubed.
(by the way the ratio of any equivalent areas on them is the scale ratio squared)

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ohhh, now i get where you got that equation from, but how do i find out what thesize of the edges are, and what does k mean?

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The volume of the small prism is (1/8) times the volume of the large prism.

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k is the scale ratio
in this case every length on the small one is 1/2 the equivalent length on the big one
so k = 1/2
and k*k*k = k^3 = 1/8

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Now I have to do a pretty long statistics problem for Sherri but will check back later.

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ok

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oh, so the length of the small one is 1/8th? but the question says they are rectangular prisms. so they will both have 2 different measurements for the 2 different sixes of the edges

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Damon,im in 8th grade math, and i don't really get what you are saying. because we haven't really talked about a scale ratio is. ill wait 'till you are done with sherri.

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OK, I am back from the statistics
Now say we have two cubes
one is 1 cm on every edge
so its volume is 1*1*1 = 1 cm^3

Now another cube (same shape of course)
But it is 2 cm on an edge
so its volume is
2 * 2 * 2 = 8 cm^3
see what happened?

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Then what if the second cube were 3 cm on every edge?
volume = 3 * 3 * 3 = 3^3

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they are not cubes. they are rectangular prisms. i really sorry if i confused you.

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It does not matter what shape, only that they are the same shape

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No matter what shape they are, every height is twice, every width is twice, every length is twice
so every volume is eight times

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Now make it harder
Let's say the first cube was 3 cm on an edge
then its volume is
3*3*3 = 27 cm^3
Now say the big one is 6 cm on an edge
its volume is
6 * 6 * 6 = 216 cm^3
BUT I would not do that
I would say
2*3 * 2*3 * 2*3 = 2^3 * 27 = 8 * 27
because my scale ratio is 2
therefore my big volume is 8 times the little one.

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no, they are rectangular prisms, not cubes

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Does not matter shape, as long as the shape is the same. Only the size changes. The volume changes as the cube of every length measurement change ratio.
here is a sphere with 2 cm radius:
vol = (4/3) pi (2^3)
here is a sphere with 4 cm radius (k = 2)
vol = (4/3) pi (4)^3
2^3 = 8
4^3 = 64
SEE - twice the radius--> EIGHT times the volume

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so wouldn't i divide the volume of the larger prism(1024 cubic feet) by 8

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YES !

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If every length on the big one were three times the little one, then the volume of the big one would be 3*3*3 = 27 times the volume of the little one.

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then i'd divide 1024 and 2, because the smaller one is half as small

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No, the volume of the little one is 1024 / 8
The smaller volume is 1/8 times the big one

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Yes!!
You got it
1024 / 8 = 128 !!

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YES!! ithank you sooooo much! i would have gotten ZAP at school if i didn't finish this homework.
THANK YOU SOOOO MUCH!!!

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You are welcome - now check that cylinder question down below. Same way.

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okay,thank you again you are pretty good at this sort of stuff

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