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7+14+21+...+2002 suma gauss

Răspuns :

[tex]\displaystyle 7+14+21+...+2002=7(1+2+3+...+286)=7 \times \frac{286(286+1)}{2} = \\ \\ =7 \times \frac{286 \times 287}{2} =7 \times \frac{82082}{2} =7 \times 41041=287287[/tex]