rumus suku ke n barisan bilangan adalah Un= 1/2 n (n-3). Nilai U12 - U 16 adalah

Un = 1/2n(n - 3)

U12 = 1/2(12)(12 - 3)
= 6 x 9
= 54

U16 = 1/2(16)(16 - 3)
= 8 x 13
= 104

U12 - U16 = 54 - 104 = -50

Un = n/2 (n - 3)
U12 = 12/2 (12 - 3)
       = 6 (9)
       = 54

U16 = 16/2 (16 - 3)
      = 8 (13)
      = 104

U12 - U16
= 54 - 104
= -50