Dik u5=8 dan u10=28 . Ditanyakan 1.beda 2. U20

U5 = a + 4b = 8
U10 = a + 9b = 28
----------------------------(-)
- 5b = - 20
b = 4

a + 4b = 8
a + 4(4) = 8
a + 16 = 8
a = 8 - 16
a = - 8

U20 = a + 19b
= - 8 + 19(4)
= - 8 + 76
= - 68

Mapel : Matematika
Bab : Barisan dan deret

Un = bn + a - b
U5 = 5b + a - b
  8 = 4b + a

Un = bn + a - b
U10 = 10b + a - b
  28 = 9b + a

8 = 4b + a
28 = 9b + a
--------------- -
-20 = -5b
  b  = -20/-5
  b  = 4

8 = 4b + a
8 = 4.4 + a
8 = 16 + a
a = 8 - 16
a = -8

Beda = 4

  Un = bn + a - b
U20 = 4.20 + -8 - 4
        = 80 - 12
        = 68