[tex]\text{Nutam cu $u(a)$ ultima cifra a numarului natural $a$.}\\
\text{Are loc proprietatea: $u(a+b)=u(a)+u(b).$}\\
x(x+1)=x^2+x\\
u(x)=0\Rightarrow u(x^2+x)=0\\
u(x)=1\Rightarrow u(x^2+x)=2\\
u(x)=2\Rightarrow u(x^2+x)=6\\
u(x)=3\Rightarrow u(x^2+x)=2\\
u(x)=4\Rightarrow u(x^2+x)=0\\
u(x)=5\Rightarrow u(x^2+x)=0\\
u(x)=6\Rightarrow u(x^2+x)=2\\
u(x)=7\Rightarrow u(x^2+x)=6\\
u(x)=8\Rightarrow u(x^2+x)=2\\
u(x)=9\Rightarrow u(x^2+x)=0\\
[/tex]
