Suku ke-5 dan suku ke-9 barisan aritmetika adalah 31dan59. Suku ke-12 adalah

U5 = a + 4b
31. = a + 4b

U9 = a + 8b
59 = a+ 8b

31 = a+4b
59 = a+8b -
------------------------
-28 = -4b
7 = b

31 = a+4b
31 = a + 7×4
31 = a + 28
3 = a

U12 = a+11b
= 3 + 11×7
= 3 + 77
= 80.

U5 = a + (n-1) b = 31
= a + (5-1) b = 31
= a + 4b = 31

U9 = a + (n-1) b = 59
= a + (9-1) b = 59
= a + 8b = 59

Eliminasi U5 dengan U9

U5 = a + 4b = 31
U9 = a + 8b = 59
___________-
-4b = -28
b = 7
Substitusi ➡ a + 4b = 31
a + 4(7) = 31
a + 28 = 31
a = 31 - 28
a = 3

Un = a + (n-1) b
U12 = 3 + (12-1) 7
= 3 + 11×7
= 3 + 77 = 80

Semoga membantu