Question

(5 + 13 + 21 + ......... + 18) = ?​

Answer 1

Correction:

5+13+21+...+181

First term = 5

Common difference = 13-5 = 8

Last term = an = 181

⇛a+(n-1)d = 181

⇛5+(n-1)(8) = 181

⇛ 5+8n-8 = 181

⇛ 8n-3 = 181

⇛ 8n = 181+3

⇛ 8n = 184

⇛n = 184/8

⇛ n = 23

Sum of the first n terms = Sn = (n/2)(a+an)

Sum of 23 terms = S 23

⇛(23/2)(5+181)

⇛ (23/2)(186)

⇛23×93

⇛2139