Question

5 + 10 + 15 + ....40 = ?

Answer 1

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

Answer 2

5 + 10 + 15 + ....40 = 
=5(1+2+..+8)
=5·8·9:2
=5·4·9
=180