u5= 15 u8=24 pada barisan bilangan aritmatika tentukan s8

U5 = 15 => a + 4b = 15
U8 = 24 => a + 7b = 24

eliminasi :
a + 7b = 24
a + 4b = 15 -
      3b = 9
        b = 9/3
        b = 3

cari nilai a
a + 4b = 15
a + 4 x 3 += 15
a + 12 = 15
       a = 15-12
       a = 3

Sn = 1/2(n) (2a+ (n-1) b)
S8 = 1/2 (8) (2 x 3 + (8-1) 3)
      = 4 (6 + 7 x 3)
      = 4 (6+ 21)
      = 4 x 27
      = 108