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# Differentiation question Watch

1. Volume of a sphere is given by formula V=4/3.pi.r^3
Find dv/dt when r=2 and dr/dt=0.4

I am not sure what do here. The answer is meant to be 6.4pi but i have no clue how to get there. If any of you have any ideas/hints that would be great!
2. (Original post by Nirm)
Volume of a sphere is given by formula V=4/3.pi.r^3
Find dv/dt when r=2 and dr/dt=0.4

I am not sure what do here. The answer is meant to be 6.4pi but i have no clue how to get there. If any of you have any ideas/hints that would be great!
The radius increases as a function of time.

what is d(r^3)/dt?

Think chain rule/implicit differentiation.

3. plug and play..
4. (Original post by Lolgarithms)
The radius increases as a function of time.

what is d(r^3)/dt?

Think chain rule/implicit differentiation.
Bit lost but i am going to try it out, thanks

(Original post by Hasufel)

plug and play..
Okay, think i might get the answer soon. Just need to figure out dv/dr
5. (Original post by Nirm)
Bit lost but i am going to try it out, thanks

Okay, think i might get the answer soon. Just need to figure out dv/dr
dV/dr should be the 'easy' bit. if V = r^3, say, then dV/dr can be found from your C1 stuff.

Note that you don't need to work out an expression for dr/dt. You can just plug in the number they give you for that when you plug in your value for r.

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