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# OCR MEI - C3 - Rates [Help Needed] watch

1. Hey all,

I was just wondering if anyone could help me on how to approach rates questions correctly? Like what's your thought process when you see this kind of question. I find these the toughest differentiation involved questions because it needs you to pick out key words.

So can anyone help ? I am happy to reply of course and answer anything.
2. (Original post by Slenderman)
Hey all,

I was just wondering if anyone could help me on how to approach rates questions correctly? Like what's your thought process when you see this kind of question. I find these the toughest differentiation involved questions because it needs you to pick out key words.

So can anyone help ? I am happy to reply of course and answer anything.
Got any example questions to go through from the textbook?
3. (Original post by 16Characters....)
Got any example questions to go through from the textbook?
Don't use the textbook so much, it's just really old and dull. How about this exam paper example:

A spherical balloon of radius r cm has volume V cm^2 , where V = 4/3.pi.r^3. The balloon is inflated constantly at a rate of 10 cm^-3 s^-1. Find the rate of increase of r when r - 8.
4. (Original post by Slenderman)
Don't use the textbook so much, it's just really old and dull. How about this exam paper example:

A spherical balloon of radius r cm has volume V cm^2 , where V = 4/3.pi.r^3. The balloon is inflated constantly at a rate of 10 cm^-3 s^-1. Find the rate of increase of r when r - 8.
This requires the use of the chain rule. We want to find out , which is given by:

Note that you can find as one over .
5. Okay so dv/dr = 4pi.r^2

Sub 8 in and that gives me 256 pi.

so 1/256pi gives like 1.24...x10-3

I assume you then just multiply by 10 as that is dv/dt ???
6. (Original post by Slenderman)
Okay so dv/dr = 4pi.r^2

Sub 8 in and that gives me 256 pi.

so 1/256pi gives like 1.24...x10-3

I assume you then just multiply by 10 as that is dv/dt ???
Yes.

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