Question

An oil slick is expanding as a circle. The radius of the circle is increasing at the rate of 20 meters per day. Express the area of the circle as a function of d, the number of days elapsed.

Answer 1

Explanation:

d = number of days since the oil slick started

r = radius in d days = 22d

A(d) = area of oil slick in d days = π(22d)2

A(d) = 484πd2

Answer 2

The circle expands it radius 20m each day. From this description you can get this equation:
r=0+ 20*d
r=20d

The oil slick expands as a circle and the formula for circle area is
`\pi` r^2. If you insert the first equation into this equation you will got:
circle area =
`\pi` r^2
circle area =
`\pi` (20d)^2
circle area =
`\pi` 400*d^2