Jumlah bilangan kelipatan 3 antara200 dan 400

201 + 204 + 207 + .... + 399

Un = 399
a +(n-1)×b = 399
201 + (n-1)×3 = 399
(n-1)×3 = 399-201
(n-1)×3 = 198
n-1 = 198/3
n-1 = 66
n = 66+1
n = 67

Sn = n/2 ×( a + Un)
S67 = 67/2 × (201 + 399)
= 67/2 × (600)
= 67/1 × 300
= 20100

semoga membantu