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5+10+15+....+40=?
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Răspuns :

Jul
Dai factor comun 5 si ai 5(1+2+3+...+8)= 5 x (8 x 9)/2 =5 x 4 x 9= 20 x 9=180
[tex]\displaystyle 5+10+15+...+40=5(1+2+3+...+8)=5 \times \frac{8(8+1)}{2} = \\ \\ =5 \times \frac{8 \times 9}{2} =5 \times \frac{72}{2} =5 \times 36=180[/tex]

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