jumlah 15 suku pertama deret arimatika

Bab Barisan dan Deret
Matematika SMP Kelas VIII

Aritmatika
U5 = 30
U10 = 50

S15 = ?

b = (U10 - U5) : (10 - 5)
   = (50 - 30) : 5
   = 4

U1 = a = U5 - 4b
            = 30 - 4 x 4
            = 30 - 16
            = 14

Sn = (n/2) x (2a + (n - 1) b)

S15 = (15/2) x (2 x 14 + (15 - 1) x 4)
       = (15/2) x (28 + 56)
       = (15/2) x 84
       = 630