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# MEI Differential Equations Question watch

1. The radius of a circular ink blot is increasing at a rate inversely proportional to its area A. Find an expression for dA/dt.

So
dr/dt = k/A

A= pir^2
dA/dr = 2pi(r)

Then what? Are we supposed to use the chain rule? Becuase I tried that and it doesn't work, the answer in the back says-
k'/rootA

HELP!!
2. (Original post by Hippysnake)
The radius of a circular ink blot is increasing at a rate inversely proportional to its area A. Find an expression for dA/dt.

So
dr/dt = k/A

A= pir^2
dA/dr = 2pi(r)

Then what? Are we supposed to use the chain rule? Becuase I tried that and it doesn't work, the answer in the back says-
k'/rootA

HELP!!
you've got

so

but so

see what you can do from there
3. The radius of a circular ink blot is increasing at a rate inversely proportional to its area A. Find an expression for dA/dt.

So
dr/dt = k/A

A= pir^2
dA/dr = 2pi(r)

Then what? Are we supposed to use the chain rule? Becuase I tried that and it doesn't work, the answer in the back says-
k'/rootA

HELP!!
Use that

and then mess about with some constants until you get the answer
4. (Original post by Chaoslord)
you've got

so

but so

see what you can do from there
Ahh thank you, I wasn't getting the answer stated by the book.

However, why is answer better than ?

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