# Help! Another differentiation problem..Watch

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4 years ago
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(Original post by iamspiderman)
The radius of a circular disc is increasing at a constant rate of 0.003cm/s. Find the rate at which the area is increasing when the radius is 20cm

I need help my teacher didn't explain properly
Observe that the radius of the circle is changing with respect to time. Can you write down what dr/dt (i.e this "rate of change") is?

Next, note the relationship between a circle's radius and its area, and recall the chain rule for differentiation:

If x and y are some functions of t then

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