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# C4 question Watch

1. How would I do part i?
I understand that dV/dt = 0.01 but I don't understand the mark scheme where it says about finding a relationship between r and h using similar triangles

Edit: the question is Q10 June 2014 OCR
2. Is this a question from an exam?
3. (Original post by IAmConfused1)
Is this a question from an exam?
Yes it's OCR June 2014 C4 Q10
4. I don't know if this will make sense, but here goes:

I subbed in first r=3 into the volume of a cone formula and then h=4.5. This gives you two equations which you then put equal to each other to find r in terms of h (r=....). This is what I did and this gave me the derivative in the mark scheme.

From there you know that dh/dV = 1/dh/dV. Then multiply dh/dV by dV/dt to get dh/dt.
5. (Original post by tryingandfailing)
I don't know if this will make sense, but here goes:

I subbed in first r=3 into the volume of a cone formula and then h=4.5. This gives you two equations which you then put equal to each other to find r in terms of h (r=....). This is what I did and this gave me the derivative in the mark scheme.

From there you know that dh/dV = 1/dh/dV. Then multiply dh/dV by dV/dt to get dh/dt.
What were the two equations you got? I know how to do everything apart from that part so I can't get to the answer:s

Edit: I got r=2h/3 by your method, thank you. Now I'm stuck further on - when i put it back into the equation i get
1/3pi(2h/3)^2 so it equals 4/27pih^3 rather than 4/9pih^3
I'm so confused

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