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    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!
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    (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.
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    \displaystyle \frac{dV}{dt}= \frac{dV}{dr} \times \frac{dr}{dt}=...

    plug and play..
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    (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)
    \displaystyle \frac{dV}{dt}= \frac{dV}{dr} \times \frac{dr}{dt}=...

    plug and play..
    Okay, think i might get the answer soon. Just need to figure out dv/dr
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    (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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