When the area in square units of an expanding circle is increasing twice as fast as its radius in linear units, the radius is

Question

(a) 1/4pi
(b) 1/4
(c) 1/pi
(d) 1
(e) pi

Answer 1

since a = pi r^2
da = 2 pi r dr
2 dr = 2 pi r dr
1 = pi r
r = 1/pi