What is the sum of the first 9 terms of an arithmetic progression if the first term is 7 and last term is 55?

Solution (By Examveda Team)

$$\eqalign{ & {S_n} = \frac{n}{2}\left( {a + l} \right) \cr & {S_9} = \frac{9}{2}\left( {7 + 55} \right) \cr & {S_9} = \frac{9}{2} \times 62 \cr & {S_9} = 9 \times 31 \cr & \,\,\,\,\,\,\,\,\, = 279 \cr} $$