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Rezolvati cu factor comun
5+10+15+20+..................+1000


Răspuns :

[tex]\displaystyle 5+10+15+20+...+1000=5(1+2+3+...+200)= \\ \\ =5 \times \frac{200(200+1)}{2} =5 \times \frac{200 \times 201}{2} =5 \times \frac{40200}{2} = \\ \\ =5 \times 20100=100500[/tex]