Question

Find S for the given arithmetic series.

a_1 = 18, d = –10, n = 12

Answer 1

I couldn't use the formulas on my own ;/

Answer 2

Well for an arithmetic series...the formula is:
S(n)=n/2{2a+(n-1)d]

so then, you just substitute the numbers in.
S(12) = 12/2 x {2x18 + (11) x -10}
..that should give you the answer.

Answer 3

thanks!