[tex]\displaystyle 10+20+30+...+120= \\\\ \texttt{Factor comun pe 10} \\ \\ 10(1+2+3+...+12)= \\ \\ \texttt{Suma lui Gaus in paranteza} \\ \\ \frac{n(n+1)}{2}, \texttt{n=ultimul nr din sir} \\ \\ \\ \frac{12(12+1)}{2}= \frac{\not12\cdot13}{\not2} =6\cdot13=78 \\ \\ 10\cdot78=780[/tex]
