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

Question

anonymous · Answer

I'll help with one.

anonymous · Answer

okay it says to find the volume of the solid

anonymous · Answer

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

anonymous · Answer

not of the top of my head

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

only if 10 is the height

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

You following so far?

anonymous · Answer

yea

anonymous · Answer

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 · Answer

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

anonymous · Answer

ya

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

idk i dont have a calculator

anonymous · Answer

oops, I did that formula wrong :/

anonymous · Answer

(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 · Answer

I left out the radius

anonymous · Answer

You get it though, right?

anonymous · Answer

kinda

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

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 · Answer

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