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r=3-1=2
an=a1+r(n-1)
an=1+2n-2=2n-1
Sn=1/2*(a1+an)*n
Sn=1/2*(1+2n-1)*n=n*n=n²
a1 = 1
an = 1 + (n - 1) * 2 = 1 + 2n - 2
an = 2n - 1 (wzór ciągu)