berapa 15 suku pertama dan deret 16+14+12+...+

Diketahui :
Deret aritmatika = 16, 14, 12, ... (beda tetap)

a = 16
b = 14 - 16 = -2

Ditanya :
S15 = ...?

Jawab :

S15 = n/2 ((2a + (n-1) b))
S15 = 15/2 ((2 . 16 + (15-1) (-2))
S15 = 15/2 ((32 + 14 . (-2))
S15 = 15/2 (32 - 28)
S15 = 15/2 (4)
S15 = 30