Question

Find the sum of the first 15 terms of Sn where
Sn = 3 + 5 + 9 + 15 + 23 + 33 +.....
(A) 1360
(B) 1460
(C) 1285
(D) 1165

Answer 1

Step-by-step explanation:

Sn = 3 + 5 + 9 + 15 + 23 + 33 +..... + tn

Sn =. 3 + 5 + 9 + 15 + 23 + 33 +..... t(n-1) + tn

Subtracting......

0 =. 3 + (2 + 4 + 6+8....+t(n-1) ) - tn

Common difference is in AP

0 = 3+ ( Sn) -tn

Solving this you get,

Sn = n/3 (n^2+8)

Put n=15

15/3(15^2+8)

5(225+8)

5(233)

$233 \times 5$

1165

Answer 2

(B). 1460

Option B is right answer.

Step-by-step explanation:

Formate of Series

$t1 = 3 \\ tn = tn - 1 \\ =2(n - 1)$