Question

Rezolvarea completa la asta:1+3+5+...+19

Answer 1

Mai intai calculezi ratia, diferenta dintre oricare 2 termeni consecutivi

3-1=2=r

Apoi calculezi numarul de termeni

n=(ultimul-primul):r+1= (19-1):2+1= 18:2+1= 10

Acum calculam

S=1+3+5+...+19

S=19+17+15+...+1

_____________________

2S=20+20+20+...+20 de 10 ori

2S= 20 x 10

2S= 200|:2

S=100