Question

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

605
660
748
816

Answer 1

you need to find the 1st term

\[T _{n} = a+(n-1)d\]

using the information for the 12th term

\[13 = a + (12 - 1)\times(-10)\]

\[13 = a - 110\]

I'll let you find a

then for the sum use

\[s _{12} = \frac{12}{2}[a + 13]\]

you found a above..

Answer 2

a = 123?

Answer 3

s12 = 738?