Question

Aflati ultima cifra a sumei 5 x n x n + 5 x n , unde "n" este numar natural oarecare .

Answer 1

 [tex]avem\;:\;5\cdot{n}^2+5\cdot{n}=5n(n+1)\;\\ unde\;n\;si\;n+1\;sunt\;numere\;naturale \;consecutive...\\ \forall\,n\in\mathbb{N}\;\;u(n\cdot(n+1))=\{0;2;6\}\\ \Rightarrow\;u(5\cdot{n}\cdot(n+1))=u\{5*0\;;\;5*2\;;\;5*6\}\,=\,0[/tex]