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C4 Rates of Change - can you explain this for me please? watch

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    I found a thread on a previous TSR post of the same thing, and a dude posted this for his written solutions:
    Attachment 727522727524

    I get his working out, but then he says dA/dt = dA/dr * dr/dt
    So then subbing in the values (fifth line), he then gets 2pi r * 4e^-λt, which is different to what he said it should be when he wrote out that dr/dt = 4λe^-λt, then when he subs dr/dt to find dA/dt, he then uses that to be 4e^-λt, missing off the λ between 4 and e.

    Can you explain that please?

    Thanks
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    (Original post by stuart4)
    Question:

    I found a thread on a previous TSR post of the same thing, and a dude posted this for his written solutions:

    I get his working out, but then he says dA/dt = dA/dr * dr/dt
    So then subbing in the values (fifth line), he then gets 2pi r * 4e^-λt, which is different to what he said it should be when he wrote out that dr/dt = 4λe^-λt, then when he subs dr/dt to find dA/dt, he then uses that to be 4e^-λt, missing off the λ between 4 and e.

    Can you explain that please?

    Thanks
    Looks like an error. The \lambda must be there.
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    (Original post by RDKGames)
    Looks like an error. The \lambda must be there.
    Thank you! I've been sitting there looking at it for about 5 minutes trying to work out what was up with it lol
 
 
 
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