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Repede va rog 5+10+15... + 300 =

Răspuns :

[tex]\displaystyle 5+10+15+...+300=5(1+2+3+...+60)= 5 \cdot \frac{60(60+1)}{2} = \\ \\ =5 \cdot \frac{60 \cdot 61}{2} =5 \cdot \frac{3660}{2} =5 \cdot 1830=9150[/tex]