Assume that the height of your cylinder is 6 inches. Consider A as a function of r, so we can write that as A(r)=2πr2+12πr. What is the domain of A(r)?

Answer 1

domain is all positive real numbers

although as a practical, realistic restriction

your cylinder has some upper size bound

maybe something like r is in the interval 0<r < 200 feet

Answer 2

There is no such thing as a negative radius, so the domain would be [0, ∞) .