Jawabanmu

2014-06-06T22:38:34+07:00
2014-06-06T22:39:18+07:00
Un = a + (n -1)b
     = Sn'
Sn =  \frac{n}{2} (a + Un)
     =  \frac{n}{2} (2a + (n - 1)b)