Hitunglah jumlah 10 suku pertama deret aritmatika 100 + 98 + 96 + 94 +...

a = 100
b = -2

 Sn = n/2 (2a + (n-1)b)
S10 = 10/2 + (2*100 + (10-1)(-2))
       = 5 (200 + (9)(-2))
       = 5 (200 + (-18))
       = 5 (188)
       = 940

jumlah 10 suku pertama = S10
beda barisan = -2
a/suku pertama = 100

Sn = n/2 (2a + (n-1) ×b)
S10 = 10/2 (2(100) + (10-1) ×-2)
S10 = 5 (200+9×2)
S10 = 5 (218)
S10 = 1090