Determine the rate of change of the area of a circle with respect to its radius.

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Lesson Parent: 
Differentiation V: Derivatives and Rates of Change
Problem: 

Determine the rate of change of the area of a circle with respect to its radius.

Answer: 

$2\pi r$ where $r$ is the radius of the circle.

SAMPLE SOLUTION

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Solution: 

Recall that the area of a circle has the formula

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From this lesson we know that the rate of change of the area with respect to the radius is $A'(r)$. Now we know that $A(r)$ is a polynomial function and that the derivative of a polynomial is known. Therefore,

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Suppose that we have a circle of radius <Sign in to see all the formulas>. The area of the circle is

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At a radius of <Sign in to see all the formulas>, $A'(r)$ is

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This is best said that at <Sign in to see all the formulas> the area of a circle will increase <Sign in to see all the formulas> per <Sign in to see all the formulas> increase in its radius when the circle is at a radius of <Sign in to see all the formulas>.