tuliskan rumus ke n dan hitung Un15 = 20,18,16,14

Arithmetic Sequence.

20, 18, 16, 14, ...
a = U₁ = 20
b = U₂ - U₁ = 18 - 20 = -2
Un = a + (n - 1)b
      = 20 - 2(n - 1)
      = - 2n + 22

U₁₅ = -2(15) + 21 = -8

a=20 b=-2
Un=20+(n-1)-2 = 20-2n+2 = -2n+22
U15=-30+22=8