calculus
posted by Taeyeon .
4. When the area in square units of an expanding circle is increasing twice as fast as its radius in linear units, the radius is
A. 1/4pi B.1/4 C.1/pi D. 1 E.pi

Area = πr²
dA/dr=2πr
dA/dt=dA/dr*dr/dt=2πrdr/dt
So when dA/dt=2dr/dt
2πrdr/dt=2dr/dt
r=1/π