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.