The Student Room Group

Algebra help

Scroll to see replies

Reply 20
Original post by Mr M
Back to the start before you made the mistake of thinking the cylinder was closed?

Edit: Do you think it is closed at one end and open at the other? The question is a bit ambiguous or have you paraphrased?


hey, i posted a pic of my working and I got to the point robbie got to

now I am asked to show that V= pi/2 (192r-r^3)

any ideas?:P
Reply 21
Original post by madfish
for the next part I am asked to show that V= pi/2 (192r-r^3)

any ideas?:s-smilie:
Find the volume for an open ended cylinder in terms h and r, but you have h so it will purely just be r's an pi's. I can't quite recall the formula, my c2 differentiation is my weakest area (mins, max peace of cake but real life applications I'm terrible at)
Reply 22
Original post by madfish
hey, i posted a pic of my working and I got to the point robbie got to

now I am asked to show that V= pi/2 (192r-r^3)

any ideas?:P
Also have you followed Mr M's instruction, my algebra was just to show you an example of how to manipulate an equation algebraically
Reply 23
Original post by Robbie242
Find the volume for an open ended cylinder in terms h and r, but you have h so it will purely just be r's an pi's. I can't quite recall the formula, my c2 differentiation is my weakest area (mins, max peace of cake but real life applications I'm terrible at)


the formula for the volume is the same with any cylinder, it is pir^h but I cannot get it in the form the are looking
Reply 24
Original post by madfish
the formula for the volume is the same with any cylinder, it is pir^h but I cannot get it in the form the are looking
Volume of a cylinder=πr2h{\pi}r^2h Your H may be wrong perhaps
Reply 25
Original post by Mr M
Try it yourself and you will see you still have R on both sides of your equation.

I am stuck at this part..
Reply 26
hhelp with this would be great, thanks :smile:
Original post by madfish
I am stuck at this part..


So it turns out you were right then, the cylinder was open at one end!

This is like a guessing game!

That does simplify to the expression you require.
Reply 28
Original post by Mr M
So it turns out you were right then, the cylinder was open at one end!

This is like a guessing game!

That does simplify to the expression you require.


I am unsure of how to get it into that form, i have tried relentlessly to get it, but i cant

my basic maths is awful :L thanks for your help, really really appreciated!
Reply 29
Original post by madfish
I am stuck at this part..

H=192ππr22πrH=\frac{192{\pi}-{\pi}r^2}{2\pi r} -> (cancel π){\pi}) H=192r22rH=\frac{192-r^2}{2r} ->(split fractions) H=1922rr22rH=\frac{192}{2r}-\frac{r^2}{2r} For fraction one simply divide by two so 1922r=96r\frac{192}{2r}=\frac{96}{r} then for the second one use the rule aman=amn\frac{a^m}{a^n}=a^{m-n}
and that should get you this equation simplified and treat a as r, I don't want to give you the entire answer
(edited 11 years ago)
Original post by madfish
I am unsure of how to get it into that form, i have tried relentlessly to get it, but i cant

my basic maths is awful :L thanks for your help, really really appreciated!


Ok right at outset you were told to cancel π\pi on the top and bottom of your expression for h. That might help you.
Reply 31
Original post by Robbie242
H=192ππr22πrH=\frac{192{\pi}-{\pi}r^2}{2\pi r} -> (cancel π){\pi}) H=192r22rH=\frac{192-r^2}{2r} ->(split fractions) H=1922rr22rH=\frac{192}{2r}-\frac{r^2}{2r} For fraction one simply divide by two so 1922r=96r\frac{192}{2r}=\frac{96}{r} then for the second one use the rule aman=amn\frac{a^m}{a^n}=a^{m-n}
and that should get you this equation simplified and treat a as r, I don't want to give you the entire answer


the second = r/2 then?
Reply 32
Original post by madfish
the second = r/2 then?
precisely, do not forget the minus from ,earlier though
Reply 33
Original post by Robbie242
precisely, do not forget the minus from ,earlier though


but sure this cannot be used in showing that that expression equals the area as we do not have 192 in it?

GOT IT!!!!!!!!!!!!!!!!! thanks robbie dude :wink:
(edited 11 years ago)
Reply 34
Original post by madfish
but sure this cannot be used in showing that that expression equals the area as we do not have 192 in it?

GOT IT!!!!!!!!!!!!!!!!! thanks robbie dude :wink:
No problem! :biggrin:

Quick Reply

Latest