Răspuns :
[tex]5+10+15+...+40=5(1+2+3+...+8)= \\ =5* \frac{8(8+1)}{2} =5* \frac{8*9}{2} =5* \frac{72}{2} =5*36=180[/tex]
[tex]5+10+15...+40= \\ = 5*(1+2+3...+8)= \\ = 5 * \frac{8*(8+1)}{2}= \\ = \frac{40*(8+1)}{2}= \\ = \frac{40*9}{2} \\ = \frac{360}{2}= \\ =\boxed{180} [/tex]
