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

1. Question:

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.

Thanks
Attached Images

2. (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.

Thanks
Looks like an error. The must be there.
3. (Original post by RDKGames)
Looks like an error. The 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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