suku ke 5 dan suku ke 9 barisan aritematika adlh 31 dan 59 ,suku ke 12 adlh

U5 = a + 4b = 31
U9 = a + 8b = 59


#

a + 4b = 31
a = 31 - 4b

subsitusi ke u9

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

a = 31 - 4(7)
a = 31 - 28
a = 3

maka U12

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