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

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 :frown:

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Reply 1
Avatar for mqb2766
mqb2766
21
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 :frown:

Do you know the formulae for the volume and surface area of a cone (Google / maths book / ...)
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
Reply 3
Avatar for mqb2766
mqb2766
21
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.
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
Reply 5
Avatar for mqb2766
mqb2766
21
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.
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
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
Reply 8
Avatar for mqb2766
mqb2766
21
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.
Original post by mqb2766
You've not substituted h=r/2 to get an equation in just r.

r = sqrt 3V/pi hx2
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.
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
Original post by joe.bbbbbbbbbbbb
oh ok

how do i solve it now though
Original post by joe.bbbbbbbbbbbb
how do i solve it now though

I've no idea what you've done?
IMG_20200919_134042.jpg
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=...
so i just need to rearrange the equation to have r on one side?
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.
Original post by mqb2766
Yes. That's what you always try to do.

ok i'll try now
would it be 2rx2 = sqrt 3V/pi

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