Odpowiedź:
an = 2n + 1
a₁ = 2 * 1 + 1 = 2 + 1 = 3
a₄ = 2 * 4 + 1 = 8 + 1 = 9
a₅ = 2 * 5 + 1 = 10 + 1 = 11
a₂₀ = 2 * 20 + 1 = 40 + 1 = 41
S₂₀ = (a₁ + a₂₀) * 20/2 = (3 + 41) * 10 = 44 * 10 = 440
S₄ = (a₁ + a₄) * 4/2 = (3 + 9) * 2 = 12 * 2 = 24
S₂₀ - S₄ = 440 - 24 = 416
Odp: suma a₅ + a₆ + a₇ + .... a₂₀ = 416
