Will someone please help me.. I don't know what i did wrong with a couple of problems please help

- anonymous

Will someone please help me.. I don't know what i did wrong with a couple of problems please help

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- schrodinger

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- anonymous

I'll help with one.

- anonymous

okay it says to find the volume of the solid

##### 1 Attachment

- anonymous

Do you know the formula for the volume of a cone?

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- anonymous

not of the top of my head

- anonymous

sorry, my computer just froze up
it is (1/3)*Pi*r^2*h

- anonymous

are 10 and 18 the heights, or are they the length of the lines they are next to?

- anonymous

yes so \[1/3\times \pi \times 5^{2}\times10\]

- anonymous

only if 10 is the height

- anonymous

\[1/3\times \pi \times 5^{2}\times18\]

- anonymous

10 is the height of the top cone and 18 is the height of the bottom cone

- anonymous

Actually, I don't think so. If you do it that way you get 733.04, which got counted wrong on your test.

- anonymous

I doubt you would get it wrong for being off by .04

- anonymous

So, lets assume that 18 and 10 are the lengths of the lines they are next to, and not the heights.

- anonymous

You following so far?

- anonymous

yea

- anonymous

Awesome. We need the heights to use our formula, so now we just need to figure out how to get them. Thinking in 3D is rather tricky, so I like to make this into something I can do in 2D.

- anonymous

Notice how we have the radius and the hypotenuse of a right triangle?

- anonymous

ya

- anonymous

Well, we can use the good old Pythagorean formula to figure out the missing side.

- anonymous

Sqrt[18^2-5^2]
and
Sqrt[10^2-5^2]
are our two heights

- anonymous

(1/3)Pi*( Sqrt[18^2-5^2] )^2
+
(1/3)Pi*( Sqrt[10^2-5^2] )^2

- anonymous

^ That should give you the correct answer, which comes out to be...

- anonymous

idk i dont have a calculator

- anonymous

oops, I did that formula wrong :/

- anonymous

(1/3)Pi*( Sqrt[18^2-5^2] )^2 + (1/3)Pi*( Sqrt[10^2-5^2] )^2
isn't right, but you get the idea

- anonymous

I left out the radius

- anonymous

You get it though, right?

- anonymous

kinda

- anonymous

(1/3)*Pi*r^2*h

- anonymous

you did it right, if 10 and 18 had been the height. Unfortunatly, they were the side length (confusing problem)

- anonymous

you just use the Pythagorean formula to get the heights, and do the same thing with them that you did with 10 and 18 the first time around.

- anonymous

Well, I'm sleepy, so I'm off to bed. I have to wake up and do calculus
D:

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