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

Thank you!! <3 Your assistance will help me graduate! ^_^

Answer 1

\(\huge S_n=\dfrac{n(a_1+a_n)}{2}\)
n is the last term of the sum, or in this case 18
a_1 is the first term in the series
a_n is the 18th term

Answer 2

and to find a_n we can use \(a_n=a_1+(n-1)d\) where d is the common difference
so \(\large a_{18}=11+(18-1)d\)