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Volumes and heights of cones.

  • Thread starter Kirito123
  • Start date
  • #1
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Homework Statement



upload_2016-5-12_18-58-44.png

Homework Equations


volume of cone = pi(r)2h / 3
volume of cylinder = pi(r)2h

The Attempt at a Solution


How can I do this, all they give is the volume and we know the value of pi ( 3.14159). I tried but you can't do it without knowing at least the radius or the height.
 

Answers and Replies

  • #2
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That is the exact wording there is nothing else to it, i might have not understood or it just doesn't make sense..
 
  • #3
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That is the exact wording there is nothing else to it, i might have not understood or it just doesn't make sense..
By just looking at the equation you have written, What do you need to do the cylinder volume in order to get the cone volume ?
 
  • #4
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you can't do it without knowing at least the radius or the height.
the height and radius is the same of both objects
 
  • #5
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yea they both have the same height and radius but i still can't find out the height or radius with just the volume.
 
  • #6
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yea they both have the same height and radius but i still can't find out the height or radius with just the volume.
look again at the equations you have written, and only at them. what is the same and what is different, type it here.
 
  • #7
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volume of cone = pi(r)2h / 3
volume of cylinder = pi(r)2h

They both use pi and the radius is squared, only different is for a cone you have to divide by 3. I'm gonna take a crack at it but it might be wrong.
So to find the radius we would do as shown below :

120 x 3 (we are reversing the original formula)

360/ 3.14 = 114.64

114.64 squared are equal to 10.707.

So the radius of both
 
  • #8
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  • #9
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don't you multiply 3.14 by 360, the reverse of that would be division.
 
  • #10
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don't you multiply 3.14 by 360, the reverse of that would be division.
no need to. we need to find the volume of the cylinder.

What happens if you multiply by 3 your first equation. don't use the 120 yet. just the first equation.
 
  • #11
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Sorry just so im clear what are you refering to when you say "first equations". Do you mean the volume of the cone or
120 x 3
 
  • #12
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Sorry just so im clear what are you refering to when you say "first equations". Do you mean the volume of the cone or
this is the first equation you have written pi(r2)2h / 3 = Volume of cone
 
  • #13
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Sorry just so im clear what are you refering to when you say "first equations". Do you mean the volume of the cone or
no numbers yet, just use the parameters, manually multiply it by 3 and type what you got
 
  • #14
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Ok just making sure so i don't mess up. If i multiply it by 3 it would become 3 Pi(h)r2??
 
  • #15
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Ok just making sure so i don't mess up. If i multiply it by 3 it would become 3 Pi(h)r2??
wasn't there a /3 aswell? where did it go?
 
  • #16
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wait doesn't the multiplied 3 cancel out the other division one?
 
  • #17
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wait doesn't the multiplied 3 cancel out the other division one?
Type it all down from the start, don't show only the result , and let's see
 
  • #18
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wait doesn't the multiplied 3 cancel out the other division one?
wait that only works if it was a division 3 that cancels a division 3.
 
  • #19
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wait that only works if it was a division 3 that cancels a division 3.
1/3 * 3 = ? any division by three can be written as x/3 = x* (1/3)
 
  • #20
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it would = 1
 
  • #21
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it would = 1
So looks like it would cancel one another. don't forget we need to multiply both sides of the equation always when we want to multiply one side we must multiply the other aswell, so equation will hold.

3* pi(r2)h / 3 = 3* Volume of cone = ?
 
Last edited:
  • #22
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sorry for late reply, anyway 3 * the volume of the cone would be 360.
 
  • #23
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since 3 x 120 = 360 as i said before right?
 
  • #24
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since 3 x 120 = 360 as i said before right?
You jump too fast to the numbers .

3* pi(r2)h / 3 = pi(r2)h = 3* Volume of cone
we multiplied by 3 so it cancels with /3, and the volume got multiplied by 3.

but what is pi(r2)h ? remember that r and h are the same
 
  • #25
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pi(r2)2h
Why did you put r2 and then 2h? and now i get what your saying, i have to find a radius and height that would be equal to 3 * volume of the cone?
 

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