Hitunglah jumlah deret aretmatika 26+21+16+11+ sampai 8 suku

11-5 = 6
6-5 = 1
1-5 = -4
-4-5 = -9

Karena rasio nya 5 maka di kurangi 5

a = 26
b = -5
[tex]s8 = \frac{n}{2} (2a + (n - 1) \times b \\ = \frac{8}{2} (2 \times 26 + (8 - 1) \times ( - 5))\\ = 4(52 + 7 \times ( - 5) \\ = 4(52 - 35) \\ = 4(17) = 68[/tex]