## anonymous 5 years ago a) Find the average rate of change of the area of a circle with respect to its radius r as r changes from 4 to each of the following. (i) 4 to 5 _______ (ii) 4 to 4.5 _______ (iii) 4 to 4.1 _______ (b) Find the instantaneous rate of change when r = 4. A'(4) = ______

1. anonymous

Formula for area: $A = \pi r^2$ To find average rate of change, find the value of A at both radii and find the difference. i.) $A _{1}=\pi (4)^2 = 16\pi$ $A _{2}=\pi (5)^2 = 25\pi$ $\Delta A = A _2 - A _1 = 25\pi - 16\pi = 9\pi$ I'll leave ii. and iii. for you to do. For part b - Find dA/dr and let r = 4. $A = \pi r^2$ $dA/dr = 2\pi r$ $dA/dr (4) = 2\pi (4) = 8\pi$

2. anonymous

Oh, whoops. For the average rate of change, you have to divide the change in area by the change in the radius. The first one is (25pi - 16pi) / (5 - 4) = 9pi / 1 = 9pi. This doesn't change the answer for the first one, but you have to remember that for the other two.

3. anonymous

thanks , I got it.