jika deret aritmatika u5 = 15 dan U9 + U23 = 96 maka S10 adalah

U5 = a + 4b
15 = a + 4b
 a  = 15 - 4b

U9 + U23 = 96
a + 8b + a + 22b = 96
2a + 30b = 96
2(15 - 4b) + 30b = 96
30 - 8b + 30b = 96
               22b = 96 - 30
                   b = 66 / 22
                      = 3
a = 15 -4b
   = 15 - 4(3)
   = 3
Un   = a + (n-1)b
U12 = 3 + (11) 3
        = 36
Sn   = n/2( a + Un)
S12 = 12/2 (3 + 36)
       = 6 (39)
       = 234