suatu barisan aritmatika 12,8,4,0,-4 nilai U9+U12 adalah

12,8,4,0,-4, -8, -12, -16, -20, -24, -28, -32, -36,
U9+U12= -20+(-32)= -52

U9=a+(n-1)b
=12+(9-1)4
=12+8.4
=12+32
=44
U12=rumus sama dg yg diatas
=12+(12-1)4
=12+11.4
=12+44
=56
U9+U12=56+44=100