5+13+21+....+181=...

materi : deret aritmatika
a = 5
b = 8
Un = 181
Sn.....?

jawab :

181 = a + bn - b
181 = 5 + 8n - 8
181 = -3 + 8n
184 = 8n
23 = n

Sn = n/2 (a + Un)
S23 = 23/2 (5 + 181)
S23 = 23/2 (186)
S23 = 23 x 93 = 2.139

maaf kalau salah, semoga membantu