Diagram shows the waves formed when a stone is thrown into the water.each wave has the shape of a circle and is $\displaystyle r cm$ away from the one before it.

a)show that the circumferences of the waves form an arithmetic progression and find the common difference.

b)If the radius if the smallest wave is $\displaystyle 4cm$ , find the radius of the tenth wave.Hence,find its circumference in term of $\displaystyle \pi$