Jawabanmu

2014-03-11T17:35:58+07:00
4. U7 - U3 = 20
...a + (7-1)b - {a + (3-1)b} = 20
..............a + 6b -{a + 2b} =20
................a + 6b - a - 2b = 20
.................................4b = 20
...................................b = 5
2014-03-11T18:45:27+07:00
U7 - U3 = 120
S8 = 116

U7 = a + 6b
U3 = a + 2b  
            4b = 120
              b = 30

Sn = n (2a + (n-1)b)
        2
S8 = 8 (2a + 7(30))
        2
116 = 4 (2a + 210)
116 = 8a + 840
8a = 116 - 840
8a = -724
a = -90,5