# Thread: Rate of Change of Area of a Circle

1. ## Rate of Change of Area of a Circle

For time t between 2 and 3 hours the radius circle C is 5t-t^2-6 feet. At time t=2.3 hours the area of the circle is (to within .001) changing at a rate of ?

How do I do this? The answers I get are always wrong.

2. Originally Posted by Velvet Love
For time t between 2 and 3 hours the radius circle C is 5t-t^2-6 feet. At time t=2.3 hours the area of the circle is (to within .001) changing at a rate of ?

How do I do this? The answers I get are always wrong.
$\displaystyle f(t) = A = \pi r^2 = \pi (5t-t^2-6)^2$

Use the chain rule to differentiate:

$\displaystyle f'(t) = 2\pi \,(5t-t^2-6) \times (5-2t)$

$\displaystyle f'(2.3) = 2\pi \,(10.6-2.3^2-6)(5-4.6) = -1.734$

(note a negative differential is fine- it means the area is getting smaller)

3. .

4. It says the answer is wrong for some reason.
I hate this online homework :/

5. I think you are lookin for dA
so; dA = A' (dt)

6. Sorry, but what does that mean? XD. My teacher doesn't teach us terms or notation for some reason

7. you know that dA/dt is the rate of change with respect to time.

so aproximately, since dA/dt= A' then dA = A' * dt you can use that to aproximate.

so you use the f'(x) the gentleman got and plug 2 for t
then the dt will be .3 since that is the change in time

8. When I plug 2 in for t, I get 0 for the answer.

9. do you have the answer?

10. With the info you gave me, I get the answer to be 0, because when i plug in 2 for t, 5t - t^2 -6 = 0.

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### rate of change of a circle

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