Question

Rezolvati urmatorul exercitu. Cu suma lui gauss

5+10+15+......................+2000=

Answer 1

Avem 5+10+15+...+2000= 5 ( 1+2+3+...+400) 

Folosind suma lui Gaus avem: 
$\frac{n(n+1)}{2}$

$5* \frac{400(400+1)}{2}$=$5* \frac{400*401}{2}= 5* 200*401 =\boxed{401000}$

Answer 2

5(1+2+3+...+400)= 5x400x501 :2= 5x200x501= 1000x501= 501000