a) Since the area of a circle is , we see that . We know the rate of change of the area, and we know the radius at that instant in time. Do you think that you can find ?

b) From a), we said that . Since we're looking for the rate of change of the diameter, we need to introduce this value into this equation. Since , we see that .

Now,

You know the rate of change of the area and you can figure out the diameter at that instant in time (since ). Can you continue and find ?