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3+6+9+...+36= suma lui gauss

Răspuns :

3(1+2+3+......+12)=

[tex]S = \frac{12* \13}{2}[/tex]

  S=78;

3 * 78  =  234;

3×(1+2+3+...+12)= 3×(12×13:2)=3×78=234