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2+4+6+8+10+12+14+16........+40


Răspuns :

[tex]\displaystyle 2+4+6+8+10+12+14+16+...+40=2(1+2+3+...+20)= \\ \\ =2 \times \frac{20(20+1)}{2} =2 \times \frac{20 \times 21}{2} =\not2 \times \frac{420}{\not2} =420[/tex]