Question

Calculati : 5+10+15+20+25+...+100 =

Answer 1

Da-mi factor comun pe 5
$S=5(1+2+3+...+20)$
Stim ca suma a n numere consecutive este
$S_{n}=1+2+..+n=\frac{n*(n+1)}{2}$
Atunci
$S=5*\frac{20*21}{2}=5*10*21=1050$

Answer 2

imparti suma la 5 
1+2+3+.......+20
20*(20+1)/2
20*21/2
420/2=210
210*5=1050(5+10+20+25+..........+100)