Question

3×1+3×3+3×5+3×7+3×9+......+3×21

Answer 1

O formula: 1+3+5+7+9+...+(2n-1)=n^2, oricare ar fi n natural nenul.

Deci 3*1+3*3+3*5+3*7+3*9+...+3*21=3(1+3+5+7+9+...+21)=3*11^2=3*121=363.

Answer 2

3•1+3•3+...+3•21=3(1+3+5+...+21); 1+3+5+...+(2n-1)=n^2; 2n-1=21=>n=11; 1+3+...+21=11^2=121; 3•121=363