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a₂₀ = a₁ + 19*4
a₂₀ = a₁ + 76
S₂₀ = (a₁ + a₂₀ ) / 2 * 20
860 = (a₁ + a₁ + 76) / 2 * 20
860 = (2a₁ + 76)/2 * 20
860 = (a₁ + 38) * 20
860 = 20a₁ + 760
100 = 20a₁
20a₁ = 100 /:5
a₁ = 5
Odp. Pierwszy wyraz ciągu wynosi 5