Odpowiedzi

2013-07-19T11:48:05+02:00

a₁ = 1

n = 20

r = 4

 

a_n = a₁ + (n-1)·r = 1 + (20-1)·4 = 1+76 = 77

 

Sn = (a₁ + a_n)·n/2

S₂₀ = (1 + 77)·20/2

S₂₀ = 780

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