What is the sum of the first 27 terms of the arithmetic sequence? −15,−11,−7,−3..

Difference between successive terms = +4

27th term =   a1 + (n-1)(d)

                     -15 + (26)(4) = 89

 

Sn = n (a₁+a₂₇)/2 = 27 (-15+89)/2 = 999