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hatsuko
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hi, can somebody check this please

the volume of a sphere is increasing at the rate of 200 cm
find in terms of pi the rate of increase of the radius when the radius is 5

dv/dt = 200

dr/dt = dv/dt x dr/dv

formula for volume of a sphere 4/3 x pi x r^2
differential = 4 x pi x r^2

200 x 4 x pi x r^2
r=5
200 x 4 x pi x 25 = 20,000 pi

seems too big to be correct..
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TenOfThem
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(Original post by hatsuko)
hi, can somebody check this please

the volume of a sphere is increasing at the rate of 200 cm
find in terms of pi the rate of increase of the radius when the radius is 5

dv/dt = 200

dr/dt = dv/dt x dr/dv

formula for volume of a sphere 4/3 x pi x r^2
differential = 4 x pi x r^2

200 x 4 x pi x r^2
r=5
200 x 4 x pi x 25 = 20,000 pi
Do you think that these are the same thing?
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hatsuko
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(Original post by TenOfThem)
Do you think that these are the same thing?
not sure, i have no idea what these things mean tbh
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TenOfThem
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(Original post by hatsuko)
not sure, i have no idea what these things mean tbh
Are you doing A Level Maths?
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hatsuko
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(Original post by TenOfThem)
Are you doing A Level Maths?
yes
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TenOfThem
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(Original post by hatsuko)
yes
In Y13?

If so

Surely you know what \dfrac{dV}{dr} and \dfrac{dr}{dV} mean

And, surely you know that the next line of working to V = \frac{4}{3} \pi r^3 should have started with one of them rather than "differential"
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hatsuko
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(Original post by TenOfThem)
In Y13?

If so

Surely you know what \dfrac{dV}{dr} and \dfrac{dr}{dV} mean

And, surely you know that the next line of working to V = \frac{4}{3} \pi r^3 should have started with one of them rather than "differential"
i am in y13, but I don't know what they mean
and i'm guessing it should be dr/dv
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TenOfThem
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(Original post by hatsuko)
i am in y13, but I don't know what they mean
Then you need to speak to your teacher

This is a basic understanding of what the derivative means
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hatsuko
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(Original post by TenOfThem)
Then you need to speak to your teacher

This is a basic understanding of what the derivative means
ill ask then, had a look on my maths but couldnt find much about it
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TenOfThem
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(Original post by hatsuko)
ill ask then, had a look on my maths but couldnt find much about it
I have a feeling that \dfrac{dy}{dx} was not well defined for you in Y12

However, it is shocking if the differential function has not been explained as a rate of change now that you are doing this work
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hatsuko
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(Original post by TenOfThem)
I have a feeling that \dfrac{dy}{dx} was not well defined for you in Y12

However, it is shocking if the differential function has not been explained as a rate of change now that you are doing this work
well, our AS teacher was pretty ... crap
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