Question

Given the arithmetic sequence –6, 1, 8…, evaluate

Answer 1

We need to find the 12th and 9th terms. Assuming -6 is the first, we get \[a_{12} = 11*7 - 6 = 71 ~~\text{and} ~~a_9 = 8*7 - 6 = 50\]
The sum becomes \[Sum = {4 \over 2}(71+50)\]

Answer 2

thanks..