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calculati suma 1+4+7+....+100

Răspuns :

Progresie aritmetica de a1=1 r=3
an=100=>100= 1+ (n-1)*3=>100=1+3n-3=>3n=102=>n=34
S34=[(a1+a34)*34]:2=>S34=[(1+100)*34]:2=3434:2=1717