Diketahui a = 13 Selisih U8-U3=10 Sn =?

Diketahui
a = 13
U8 - U3 = 10
a + 7b - a + 2b = 10

Penyelesaian
13 + 7b - 13 + 2b = 10
                   0 + 5b = 10
                          5b = 10
                            b = 10/5
                            b = 2

Sn = n/2 (2a + (n - 1) b)
      = n/2 (2 (13) + (n - 1) 2)
      = n/2 (26 + 2n -2)