suatu barisan aritmatika dgn U3=8 dan U7=20 hasil dari U12+U18 adalah tolong

u3 = a+2b = 8
u7 = a+ 6b = 20
---------------------- -
-4b = - 12
b = 3

u3 = a+2(3) = 8
u3 = a+ 6 = 8
a = 8-6
a= 2

dit u12+u18

u12 = a+11b
u 12 = 2 + 11(3)= 2+33 = 35

u 18 = a+17b
= 2 +17(3) = 2 + 51 = 53

u12+u18 = 35+51 = 86