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Aratati ca pt. orice nr. natural x, nr.: x*x+9x+20=cu produsul a doua nr consecutive

Răspuns :

[tex]x^2+9x+20=\\ x^2+4x+5x+20=\\ x(x+4)+5(x+4)=(x+4)(x+5)\\ 4,5\ sunt\ consecuive,deci\ cerinta\ este\ adevarata.[/tex]