Arithmetic sequences

Question

A number of interlocking rings each 1 cm thick are hanging from a peg. The top ring has an outside diameter of 20 cm. The outside diameter of each of the outer rings is 1 cm less than that of the ring above it. The bottom ring has an outside diameter of 3 cm. What is the distance from the top of the ring to the bottom of the bottom ring.

Answer 1

3,4,5,...20 are anAP with d=1, a1=3,,a17=20

s17 =(n-1)/2 x (a1+a17)= 16/2(3+20)= 8x23= 184 cm