Question

What is the sum of the first 18 terms of the arithmetic series
11 + 15 + 19 + 23 + ⋯?

a.) 684
b.) 810
c.) 831
d.) 846

Answer 1

is it 810 ??

Answer 2

u see the first term i.e a_1 = 11
and common difference i.e d = 4
and sum of "n" terms is
\[S_n = \frac{ n }{ 2 }(2a_1 + (n - 1)d)\]