Question

Find S11 for 3 + 1 + (-1) + (-3) +…

Answer 1

If you mean to say to find the 11th term, then know that this is an arithmetic sequence (where you have a common difference). 

The common difference in this case is -2. 

The formula for finding any term in an arithmetic sequence (specifically the explicit formula) is: $a_n = a_1 + (n-1)d$ where a_n is the term, a_1 is the first term, n is the term number (position) and d is the common difference. 

Let's substitute the corresponding values to the formula.

$a_{11} = 3 + (11-1)-2 a_{11} = 3 + (10)-2 a_{11} = 3 - 20 a_{11} = -17$

The 11th term is -17.