Jawabanmu

2014-06-02T07:40:05+07:00
5, 8, 11, ....
a = 5; b = 8 - 5 = 3
S15 = 15/2 (2a + 14b) = 15/2 (10 + 42) = 390
2014-06-02T07:41:50+07:00
Sn = n/2 (2a + (n-1) b)
= 15/2 (2.5 + (15-1) 3)
= 15/2 (10 + 14.3)
= 15/2 (10+ 42)
15/2 . 52
390