(a) Find the average rate of change of the area of a circle with respect to its radius r as r changes from 2 to each of the following. (i) 2 to 3
(ii) 2 to 2.5
(iii) 2 to 2.1
(b) Find the instantaneous rate of change when r =2.

ai) 5pi

aii) 4.5pi

aiii) 4.1pi

b) 4pi

Step-by-step explanation:

a) Area of a circle is given by pi×r^2.

The average rate of change of the area of a circle from r=b to r=a is (pi×b^2-pi×a^2)/(b-a).

Let's simplify this.

Factor pi from the terms in the numerator:

pi(b^2-a^2)/(b-a)

Factor the difference of squares in the numerator:

pi(b-a)(b+a)/(b-a)

"Cancel" common factor (b-a):

pi(b+a).

So let's write a conclusive statement about what we just came up with:

The average rate of change of the area of a circle from r=b to r=a is pi(b+a).

i) from 2 to 3 the average rate of change is pi(2+3)=5pi.

ii) from 2 to 2.5 the average rate of change is pi(2+2.5)=4.5pi.

from 2 to 2.1 the average rate of change is pi(2+2.1)=4.1pi.

b) It looks like a good guess at the instantaneous rate of change is 4pi following what the average rate of change of the area approached in parts i) through iii) as we got closer to making the other number 2.

Let's confirm by differentiating and then plugging in 2 for r.

A=pi×r^2

A'=pi×2r

At r=2, we have A'=pi×2(2)=4pi. It has been confirmed.