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15+21+27+...+93=????!

Răspuns :

15+21+27+...+93=progresie aritmetica
an=a1+(n-1)×r
93=15+(n-1)×6
93=15+6n-6
93-9=6n
n=14
S= n×[(a1+an)/2]=14×[(93+15)/2]=14×(108:2)=14×54=756
S=756