[tex]5) \displaystyle \\
2013 + 2(1+2+...+2012) = \\ \\
= 2013 + \frac{2 \times 2012(2012+1)}{2}= \\
= 2013+2012 \times 2013 =2013(1+2012) = 2013 \times 2013 = \boxed{2013^2=pp}[/tex]
[tex]11) \\
a) ~~~18 + 20 -28 = 38-28 = \boxed{10\;elevi~participa~la~ambele~activitati} \\ \\
b)~~~ 18 - 10 = \boxed{8\;elevi~participa~numai~la~cercul~de~biologie}[/tex]
