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a5=5*5 - 3 = 25-3=22
a20=5*20 - 3= 100-3=97
Sn=a1+an/2 * n
Sa₅-a₂₀= ((a₅+a₂₀)/2)* 16= (22+97)/2 * 16=119/2 * 16= 59,5*16=952
Suma ciagu od a₅do a₂₀ wynosi 952