Find the sum of the following (iii) 6 + 13 + 20 +...........+ 97

Find the sum of the following
(iii) 6 + 13 + 20 +...........+ 97
Solution :
l = 97, a = 6, d = 13 - 6  = 7
From this, we have to find the number of terms.
n = [(l - a)/d] + 1
n = [(97 - 6)/7] + 1
  =  (91/7) + 1
n  = 13 + 1
n = 14
Sn  = (n/2) [a + l]
  =  (14/2)[6 + 97]
  =  7[103]
  =  721
Hence the sum of given series is 721.