Jawabanmu

2014-01-27T20:56:33+07:00
Un = a+(n-1)b
U5 = a+4b=18
U9 = a+8b=34
      (dieliminasi)
       -4b = -16
          b = 4
a+4(4) = 18
a+16 = 18
a = 2

S20 = 20/2 (2.2 +(20-1)4
       = 10 (4+76) = 800
 
2014-01-27T21:00:10+07:00
u5 = a+4b = 18
u9 = a+8b = 34
dieliminasi

ketemu a= 2, b= 4
S20 = 1/2.20(2.2+19.4)
= 10.80 = 800