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Calculati: 5+10+15+.......+625

Răspuns :

[tex]\displaystyle 5+10+15+...+625=5(1+2+3+...+125)= \\ \\ =5 \cdot \frac{125(125+1)}{2} =5 \cdot \frac{125 \cdot 126}{2} =5 \cdot \frac{15750}{2} =5 \cdot 7875=39375[/tex]