Question

What is the sum of the arithmetic sequence 3, 9, 15…, if there are 22 terms?

Answer 1

find A_22 first

\[A_{22} = 3 + (22 - 1)6\] \[A_{22} = 3 + 126 = 129\]

now that you have the last term solve for sum

\[S_n = \frac{n(a_n + a_1)}{2}\]
\[S_n = \frac{22(129 + 3)}{2}\] can you do it from there?

Answer 2

btw...the formula for A_n is \[A_n = A_1 + (n-1)d\]