4) 12. Hitunglah jumlah semua bilangan kelipatan 4 antara 12 dan 120! ​

Penyelesaian

Kelipatan 4 antara 12 dan 120

16, 20, 24, … 116

U₁ = 16

b = 20 – 16 = 4

U(?) = 116

• mencari n

Uₙ = a + (n – 1)b

116 = 16 + (n – 1)(4)

116 – 16 = 4n – 4

100 = 4n – 4

100 + 4 = 4n

4n = 104

n = 104 ÷ 4

n = 26

• S₂₆

Sn = n/2 (2a + (n – 1)b)

S₂₆ = 26/2 (2(16) + (26 – 1)(4))

S₂₆ = 13 (32 + (25)(4))

S₂₆ = 13 (32 + 100)

S₂₆ = 13 (132)

S₂₆ = 1.716