Suku ke -5 dan suku ke -9 barisan aritmatika adalah 31 dan 59. Suku ke -12 adalah….

U5 = 31 => a + 4b = 31
U9 = 59 => a + 8b= 59
—————— —
-4b = -28
b = -28/-4
b = 7
subtitusi b = 7
a + 4b = 31
a + 4(7) = 31
a + 28 = 31
a = 31 – 28
a = 3

U12 = ?? => a + 11b =??

U12 = a + 11b
U12 = (3) + 11 (7)
U12 = 3 + 77
U12 = 80

smoga brmanfaat.