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2005+2*(1+2+3+......2004) este pătrat perfect.......cine il explica???

Răspuns :

1+2+3+......+2004=2004*2005:2asta reiese din formula lui Gauss 1+2+3+...+n=n*(n+1):2


2005+2*(1+2+3+......+2004)=2005+2*2004*2005:2=2005+2004*2005=2005*(1+2004)=2005*2005=2005²  deci este patrat perfect
[tex]2005+2*(1+2+3+...+2004)=\\ 2005+\not2*\frac{2004*2005}{\not2}=\\ 2005+2004*2005=\\ 2005(1+2004)=2005^2[/tex]