Jumlah deret aritmetika berikut: 70+66+62+58+....+6=

a = 70
b = -4

Un = a + (n-1)b
6 = 70 + (n-1)-4
6 = 70 + -4n + 4
6 = -4n +74
6-74 = -4n
-68 = -4n
n = 17

Sn = n/2(a + Un)
S17 = 17/2(70+6)
S17 = 17/2(76)
S17 = 646

Maaf kalau salah.