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

15=11+4
19=15+4
23=19+4


so they are arithmetic sequence
a1=11
r=4
a(n)=a1+(n-1)r=11+(n-1)4=11+4n-4=4n+7

s(n)=1/2 *n(a1+an)

Answer 2

Wow ok I'm stupid I just realized it has a pattern format, and just checking @amoodarya it's 810?

Answer 3

s(18)=18/2 (a1+a18 )
=9(11+11+(18-1)4)
=9(22+68)=9(90)=810

Answer 4

YES, thanks mate!