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3°1+3°3+3°5+3°7+3°9+....+3°21. °=ori

Răspuns :

3°1+3°3+3°5+3°7+3°9+....+3°21=
1*(1+3+5......+21)=  n*n, unde n se gaseste in formula ( 2*n-1)= ultimul termen=21
21=2*n-1= 2*11 - 1 deci n=11

1*(1+3+5......+21)= 11*11=121