Find the sum of the first 20 terms of the arithmetic sequence - 16, - 20, - 24?

An = a1 + (n - 1) * d

a20 = - 16 + (20 - 1) * - 4

a20 = - 16 + (19 * - 4)

a20 = - 16 - 76

a20 = - 92

sn = n (a1 + a20) / 2

s20 = 20 (-16 - 92) / 2

s20 = 20 (-108) / 2

s20 = - 2160/2

s20 = - 1080 < = = the sum of the first 20 terms