Question

What is the sum of a 12–term arithmetic sequence where the last term is 13 and the common difference is –10?

is it 816?

Answer 1

\[13 = a + 11*(-10)\]
\[a = 13 + 110 = 123\]
\[S_{12} = \frac{12}{2} (123+ 13)\]
\[S_{12} = 6 \times 136 \]

Answer 2

thank you