# Geometry [Solved!]

**phinah** 06 Nov 2019, 16:41

### My question

Exercise #1 on the url below

### Relevant page

Geometry and Modeling Practice - MathBitsNotebook(Geo - CCSS Math)

### What I've done so far

My solution is not listed in the possible choices.

`2(13)(75)(3)pi` cubic inches / (12 in x 12 in x 12 in) = 10.6 cubic feet.

X

Exercise #1 on the url below

Relevant page
<a href="https://mathbitsnotebook.com/Geometry/Modeling/MDModelingPractice.html">Geometry and Modeling Practice - MathBitsNotebook(Geo - CCSS Math)</a>
What I've done so far
My solution is not listed in the possible choices.
`2(13)(75)(3)pi` cubic inches / (12 in x 12 in x 12 in) = 10.6 cubic feet.

## Re: Geometry

**Murray** 06 Nov 2019, 19:37

It appears you have found the inner radius (`2xxpixx13`) then multiplied by the height (`75`) and by the thickness (`3`). That makes sense (kind of) but what about if we considered it from the outer radius, that is:

`2xxpixxrxxhxxd = 2xxpixx16xx75xx3?`

Our answer is a lot bigger. (I suspect this is how most tradesmen would calculate it - it is greater than the required amount, so there is some "safety margin" leftover.)

The way to approach this problem is to find the volume of the whole thing (heater plus insulating sleeve) minus the volume of the heater.

X

It appears you have found the inner radius (`2xxpixx13`) then multiplied by the height (`75`) and by the thickness (`3`). That makes sense (kind of) but what about if we considered it from the outer radius, that is:
`2xxpixxrxxhxxd = 2xxpixx16xx75xx3?`
Our answer is a lot bigger. (I suspect this is how most tradesmen would calculate it - it is greater than the required amount, so there is some "safety margin" leftover.)
The way to approach this problem is to find the volume of the whole thing (heater plus insulating sleeve) minus the volume of the heater.

## Re: Geometry

**phinah** 13 Nov 2019, 08:50

`[pi times (16^2) times 75] - [pi times (13^2) times 75]` divided by the cubic factor = 11.9!!

Thank you!!

X

`[pi times (16^2) times 75] - [pi times (13^2) times 75]` divided by the cubic factor = 11.9!!
Thank you!!

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