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r=3
an=a1+(n-1)*r
-7+3n-3=227
3n=227+10
n=237/3
n=79
Sn=(a1+an)*n/2
S79=(-7+227)*79/2 =220*79/2 =110*79
S79=8690
wzór na sume n poczatkowych wyrazów:
trzeba wyznaczyć n
227 = 3n-10
stąd
n=79
więc