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1. Maths Q help - urgent
The diagram shows a solid metal cylinder. (you dont need the diagram)
The cylinder has base radius 2x and height 9x.
The cylinder is melted down and made into a sphere of radius .
Find an expression for r in terms of x.
Last edited by DeathByBSOD; 02-07-2012 at 20:57.
2. Re: Maths Q help - urgent
(Original post by DeathByBSOD)
The diagram shows a solid metal cylinder. (you dont need the diagram)
The cylinder has base radius 2x and height 9x.
The cylinder is melted down and made into a sphere of radius .
Find an expression for  in terms of x.
Work out the volume, set it = volume of sphere

Solve for (whatever letter you've missed out when copying).
3. Re: Maths Q help - urgent
(Original post by hassi94)
Work out the volume, set it = volume of sphere

Solve for (whatever letter you've missed out when copying).
sorry! its was Find an expression for r in terms of x.
4. Re: Maths Q help - urgent
(Original post by cube123)
Volume of cylinder = (2x)^2 * pi * 9x = 36x^3 * pi. Volume of a general sphere with radius r = (4/3) * pi * r^3. Therefore as the volume of the new sphere is 36x^3 * pi we know that 36x^3 * pi = (4/3) * pi * r^3. Divide both sides by (4/3)*pi. This gives us the following: 27x^3 = r^3. Cube root both sides to get 3x=r.
Thank you ever so much =)
5. Re: Maths Q help - urgent
(Original post by DeathByBSOD)
Thank you ever so much =)
You're welcome
6. Re: Maths Q help - urgent
(Original post by cube123)
Volume of cylinder = (2x)^2 * pi * 9x = 36x^3 * pi. Volume of a general sphere with radius r = (4/3) * pi * r^3. Therefore as the volume of the new sphere is 36x^3 * pi we know that 36x^3 * pi = (4/3) * pi * r^3. Divide both sides by (4/3)*pi. This gives us the following: 27x^3 = r^3. Cube root both sides to get 3x=r.
Hi, how does dividing both sides by (4/3)*pi give : 27x^3 = r^3 ?
7. Re: Maths Q help - urgent
(Original post by cube123)
...
The forum guidance asks that we do not post full solutions

8. Re: Maths Q help - urgent
(Original post by TenOfThem)
The forum guidance asks that we do not post full solutions

So you can't tell me? it seems completely counter-intuitive!

If you divide anything by 4/3 your surely going to get a number bigger than what you started with right?
9. Re: Maths Q help - urgent
(Original post by Tronick)
So you can't tell me? it seems completely counter-intuitive!

If you divide anything by 4/3 your surely going to get a number bigger than what you started with right?
4/3 is bigger than 1

if you divide by it how will the number get bigger
10. Re: Maths Q help - urgent
(Original post by TenOfThem)
4/3 is bigger than 1

if you divide by it how will the number get bigger
Ok, so then how would you work it out?

I tried (36/3)*4=48, obviously not correct
11. Re: Maths Q help - urgent
scrap that, got it
12. Re: Maths Q help - urgent
(Original post by Tronick)
So you can't tell me? it seems completely counter-intuitive!

If you divide anything by 4/3 your surely going to get a number bigger than what you started with right?
The point of this forum is to teach people maths, not give them the answer. If we just gave you the answer you wouldn't learn how to do it, and you wouldn't be able to solve other problems.
13. Re: Maths Q help - urgent
(Original post by ThatPerson)
The point of this forum is to teach people maths, not give them the answer. If we just gave you the answer you wouldn't learn how to do it, and you wouldn't be able to solve other problems.
Tronik is not the OP

He was just asking a simple question
14. Re: Maths Q help - urgent
(Original post by TenOfThem)
Tronik is not the OP

He was just asking a simple question

But before that he said:

(Original post by Tronick)
So you can't tell me? it seems completely counter-intuitive!
Which was in response to your post about the forum rules that say we can't post full solutions.