Narisan aritmatika U2=7 dan U6=19 suku ke8 adalah

u2= a+b=7
u6= a+5b = 19
U6-u2= 4b=12
b=3

a=7-3=4

U8= a+7b = 4+ 7*3 =25

Un = a + (n-1)b

U2 = 7
a + b = 7

U6 = 19
a + 5b = 19

a + 5b = 19
a + b = 7
--------------- -
4b = 12
b = 12/4
b = 3

a + b = 7
a + 3 = 7
a = 7 - 3
a = 4

U8 = a + 7b
      = 4 + 7(3)
      = 4 + 21
      = 25