Question

Ex. 15 va rog.........

Answer 1

$a=\sqrt{|-2-4-6-...-2n|-|-n|}\\\\a=\sqrt{|-2(1+2+3+...+n)|-n}\\\\a=\sqrt{2(1+2+3+...+n)-n}\\\\a=\sqrt{\frac{2n*(n+1)}{2}-n}\\\\a=\sqrt{n*(n+1)-n}\\a=\sqrt{n^2+n-n}\\a=\sqrt{n^2}\\a=|n|=n,\ care\ este\ natural\ deoarece\ n\ este\ natural.$