Jumlah deret aritmatika 50 + 49 1/2 + 49 + ....... + (-20)

50 + 49 1/2 + 49 + ....... + (-20)

n = 2 (50 + 20) = 140
b = 49,5 - 50 = -0,5

Sn = ½n [2 x U1 + (n - 1)b]
S140 = ½ x 140 [2 x 50 + (140-1) x 0,5]
= 70 [100 + (-69,5)]
= 70 (30.5)
= 2135