Find the sum of the following

(i) 3, 7, 11,… up to 40 terms.

Solution :

Number of terms (n) = 40

First term (a) = 3

Common difference (d) = 7 - 3 = 4

Sn = (n/2) [2a + (n - 1)d]

= (40/2) [2(3) + (40 - 1)4]

= 20 [6 + 156]

= 3240

Hence the sum of 40 terms of the given series is 3240.