Jawabanmu

2014-02-09T10:23:35+07:00
Un= a+(n-1)b
U9= 12+(9-1)-4= 12+8.-4= 12+(-24)= 12-24= -12
U12= 12+(12-1).-4= 12+(11.-4)= 12+(-44)= -32
jadi u9+u12 adalah -12+(-32)= -12-32= -44
2014-02-09T10:35:45+07:00
12,8,4,0,-4,...

12 = 4.3
8   = 4.2
4   = 4.1
0   = 4.0 
-4  = 4.(-1) 

Un = 4(4-n)

U9 + U12 = ?
U9  = 4(4-9)
      = 4.(-5)
      = -20 

U12 = 4(4-12)
       = 4.(-8)
       = - 32 

U9 + U12 = -20 + -32 
               = - 52 

Semoga membantu ^_^