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

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