Jawabanmu

2014-01-31T11:15:45+07:00
Un = a + (n-1)b

U₃ + U₇ = 20
(a + 2b) + (a + 6b) = 20
2a + 8b = 20

U₉ - U₂ = 21
(a + 8b) - (a + b) = 21
7b = 21
b = 3

2a + 8b = 20
2a + 8(3) = 20
2a + 24 = 20
2a = -4
a = -2

U₂₀ = a + 19b
      = -2 + 19(3)
      = -2 + 57
      = 55

1 2 1
2014-01-31T13:30:38+07:00
U9 - U2 = 21
a+8b - (a+b) = 21
a+8b-a-b = 21
7b = 21
b = 3
U3 + U7 = 20
a+2b+a+6b = 20
2a+8b = 20
2a + 8*3 = 20
2a = 20-24
a = -4/2 = -2
U20 = a+19b
       = -2+19*3
       = -2+57
       = 55