Tentukan jumlah deret aritmetika berikut: 1. 80 + 70 + 60 + …… sampai 12 suku 2. 2 + 3 + 4 + ……….. sampai 40 suku

1. a = 80
u2 = a + b
70 = 80 + b
-10 = b
s12 = 12/2 (2a + (n-1)b)
s12 = 6 (2*80 + 11*(-10)
s12 = 6 (160 - 110)
s12 = 6 * 50
s12 = 300

2. a = 2
u2 = a+b
3 = 2+b
1 = b
s40 = 40/2 (2*2 + (40-1)1)
s40 = 20 (4 + 39)
s40 = 20 * 33
s40 = 660

Semoga dapat membantu ^o^