A stone drops into a pond, creating a circular ripple. The radius of the ripple increases from 0 cm, at a constant rate of .

At what rate is the area enclosed within the ripple increasing when the radius is 15 cm?

A.     

B.     

C.     

D.     

A spherical raindrop of radius metres loses water through evaporation at a rate that depends on its surface area.  The rate of change of the volume of the raindrop is given by

, 

where is time in seconds and is the surface area of the raindrop. The surface area and the volume of the raindrop are given by    and    respectively.

1. Show that    is constant.   (1 mark)
   

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2. How long does it take for a raindrop of volume   m³  to completely evaporate?     (2 marks)
   

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