diketahui barisan bilangan U5 = 12 dan U9 = 30 . tentukan suku ke - 12

Eliminasi a
U9 = a + 8b = 30
U5 = a + 4b = 12
------------------------ -
4b = 18
b = 18/4 = 9/2

Subtitusi nilai b ke salah satu persamaan
a + 4b = 12
a + 4.9/2 = 12
a + 18 = 12
a = 12 - 18 = -6

Un = a + (n - 1)b
U12= a + 11b
U12= -6 + 11.9/2
U12= -12/2 + 99/2
U12= 87/2

Semoga membantu ^-^