Jawabanmu

2014-04-14T11:10:26+07:00
U₅ = a + (5 - 1)b = 18
     = a + 4b = 18
U₉ = a + (9 - 1)b = 34
     = a + 8b = 34

a + 8b = 34
a + 4b = 18  -
      4b = 16
        b = 4

a + 4b = 18
a + 4(4) = 18
a + 16 = 18
         a = 18 -16 = 2

Sn =  \frac{n}{2} ( 2a + ( n - 1) b )
S₂₀=  \frac{20}{2} (2 (2) + ( 20 - 1) 4)
     = 10 (4 + 76)
     = 10 x 80 = 800

maaf ya kalo ada yang salah...