Jawabanmu

2014-02-17T09:17:51+07:00
U5 = a + 4b = 12 u8 = a + 7b = 96 3b = 84 b = 84 : 3 = 28 a + 4b = 12 a + 4.28 = 12 a + 112 = 12 a= 12 - 112 a= -100 cara cari jumlah nya gk tau.
2014-02-17T14:59:35+07:00
U₅ = 12 =  a + 4b
U₇ = 96 = a + 7b
 U₇ - U₅ : a + 7b – a + 4b = 96 – 12
a – a + 7b – 4b = 84
 0 + 3b = 84
 b = 84 : 3

 b = 28


 a + 4b =12

 a + 4 . 28 = 12

 a + 112 = 12

 a            = 12 – 112
=  -10
Sn = ½ n ( a + Un ) = ½ n (a + a + (n-1) b) = ½ n (2a + (n-1) b)
S₈ = ½ . 8 ( 2 . -100 + (8 - 1) . 28) = 4 ( -200 + 7 . 28 ) = 4 ( -200 + 196 )  = 4 . -4   = -16