Jawabanmu

2014-04-27T09:12:17+07:00
Setiap naik = +3 angka

jadi 20 = (3x8) + 84 = 108
2014-04-27T09:18:09+07:00
Un = a + (n-1) b
U8 = a + 7b = 72 (persamaan 1)
U12 = a + 11b = 84 terus dikurangi
hasilnya -4b = -12 jadi b = 3
masuk ke persamaan 1. a + 7b = 72
a + 7(3) = 72
a = 72 - 21 = 51

Sn = n/2 (2a + (n-1)b)
S20 = 20/2 (2x51 + (19x3))
=10 (102 + (57)
= 10 (159)
= 1590

Semoga Membantu.