Jawabanmu

2014-07-03T10:44:22+07:00
U3 = a + (n-1) b
1/8 = a + (1/8-1) b
1/8= a - 7/8 b

U7= a + (n-1) b
2 = a + (2-1) b
2= a+b

Eliminasi

a + b =2
a -7/8b = 1/8

hasilnya 15/8b=15/8
b=1
a=1

Jumlah suku ke 7

S7 = n/2 (2a+(n-1) b
= 7/2 ( 2.1 + (7-1) .1
= 7/2 (8)
= 28
2014-07-03T10:45:36+07:00
U7 / U3
 2 / 1/8 = ar^6 / ar^2
16 = r^3
r = ∛16 = 2.5
u7 = ar^6
2 = a (2,5)^6
2 = a * 244.14
a = 0.008
Sn = a(r^n -1)/ r - 1
     = 0,008 (2,5^7 - 1) / 1,5 - 1
insya Allah caranya seperti itu, maaf klao seandainya salah.
ini lagi buru-buru jd nggak bs nyelesaiin.