4.
[tex]\it 1\longrightarrow9 \Rightarrow9\ cifre\\ \\ 10\longrightarrow 99 \Rightarrow 2\cdot90=180\ cifre\\ \\ 100\longrightarrow114 \Rightarrow 3\cdot15=45\ cifre\\ \\ 9+180+45=234\ cifre[/tex]
5.
[tex]\it =(165\cdot3-154):31=(495-154):31=341:31=11[/tex]
6.
106 + (106-29 )= 106 + 77 = 183km (în primele 2 zile)
183 + 183 = 366km (în cele 3 zile)
8.
[tex]a+b=17= \begin{cases} 1+16\\2+15\\3+14\\4+13\\5+12\\6+11\\7+10\\8+9\end{cases} \Rightarrow 8\cdot9=72 (produsul \ maxim) \Rightarrow (a,b)\in\{(8,\ 9),(9,8)\}[/tex]
9.
[tex]\left.\begin{aligned} a-b=c\Rightarrow a=b+c\Rightarrow b+c=a\\ \\ a+b+c=2012\ \ \ \ \ \ \ \end{aligned}\right\}\Rightarrow a+a=2012=1006+1006 \Rightarrow a=1006[/tex]
