[tex]5+10+15+...+40 = \\ \\ =5\times(1+2+3+...+ 8) =\\\\ =5\times\dfrac{8\times (8+1)}{2} = \\ \\ = 5\times \dfrac{8\times 9}{2}=\\ \\ =5\times 4\times 9= \\ \\ =180 \\ \\ \\ \boxed{1+2+3+...+n = \dfrac{n\times(n+1)}{2}}\rightarrow $ Suma lui Gauss[/tex]
