Odpowiedź:
a₁ = 2
r = 5
a₂₀ = a₁ + 19r = 2 + 19 * 5 = 2 + 95 = 97
S₂₀ = (a₁ + a₂₀) * 20/2 = (2 + 97) * 10 = 99 * 10 = 990
