A cone has a volume of 562.5 pi cm^3 PLS HELPPP

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joe.bbbbbbbbbbbb
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A cone has a volume of 562.5 pi cm^3
The radius of the base of the cone is equal to twice the height of the cone
Work out the cufrved surface area of the cone
Answer correct to 3 significant figures
i'm very stuck
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mqb2766
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(Original post by joe.bbbbbbbbbbbb)
A cone has a volume of 562.5 pi cm^3
The radius of the base of the cone is equal to twice the height of the cone
Work out the cufrved surface area of the cone
Answer correct to 3 significant figures
i'm very stuck
Do you know the formulae for the volume and surface area of a cone (Google / maths book / ...)
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joe.bbbbbbbbbbbb
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(Original post by mqb2766)
Do you know the formulae for the volume and surface area of a cone (Google / maths book / ...)
yeah but i have no clue how to use it in this question
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mqb2766
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(Original post by joe.bbbbbbbbbbbb)
yeah but i have no clue how to use it in this question
You know h=r/2 and V=562.5. That means you can find the radius of the base (and hence height) and so plug into the surface area formula.
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joe.bbbbbbbbbbbb
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(Original post by mqb2766)
You know h=r/2 and V=562.5. That means you can find the radius of the base (and hence height) and so plug into the surface area formula.
how do i find the radius of the base from that tho
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mqb2766
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How about writing down the volume formula and using the two facts mentioned. You have a single equation in r which can be solved. If you're having difficulty, upload what you've done and state clearly what you're having problems with.
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joe.bbbbbbbbbbbb
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(Original post by mqb2766)
How about writing down the volume formula and using the two facts mentioned. You have a single equation in r which can be solved. If you're having difficulty, upload what you've done and state clearly what you're having problems with.
i'm confused
dont u need height number to solve radius
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joe.bbbbbbbbbbbb
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(Original post by mqb2766)
How about writing down the volume formula and using the two facts mentioned. You have a single equation in r which can be solved. If you're having difficulty, upload what you've done and state clearly what you're having problems with.
radius is sqrt 3V/pi h
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mqb2766
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(Original post by joe.bbbbbbbbbbbb)
radius is sqrt 3V/pi h
You've not substituted h=r/2 to get an equation in just r.
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joe.bbbbbbbbbbbb
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(Original post by mqb2766)
You've not substituted h=r/2 to get an equation in just r.
r = sqrt 3V/pi hx2
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mqb2766
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(Original post by joe.bbbbbbbbbbbb)
r = sqrt 3V/pi hx2
It would help to see your working. If you replace h with r/2, there will be no h in this equation. That is the whole point of doing it.
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joe.bbbbbbbbbbbb
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(Original post by mqb2766)
It would help to see your working. If you replace h with r/2, there will be no h in this equation. That is the whole point of doing it.
oh ok
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joe.bbbbbbbbbbbb
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(Original post by joe.bbbbbbbbbbbb)
oh ok
how do i solve it now though
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mqb2766
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(Original post by joe.bbbbbbbbbbbb)
how do i solve it now though
I've no idea what you've done?
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joe.bbbbbbbbbbbb
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mqb2766
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You can't solve that as r appears on both sides of the equation. Also you're told what V is?
Do the substitution h=r/2 in the V=... formula before you rearrange the equation to get r=...
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joe.bbbbbbbbbbbb
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so i just need to rearrange the equation to have r on one side?
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(Original post by joe.bbbbbbbbbbbb)
so i just need to rearrange the equation to have r on one side?
Yes. That's what you always try to do.
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joe.bbbbbbbbbbbb
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(Original post by mqb2766)
Yes. That's what you always try to do.
ok i'll try now
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joe.bbbbbbbbbbbb
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#20
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would it be 2rx2 = sqrt 3V/pi
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