Răspuns:
2-3+405+...+2020
S1=2+4+...+2020=
2(1+2+...+1010)/2=1010*1011=
S2=-3-5-,..-1019 adui si scazi 1
S2= -1-3-5-...-1019+1=
-(1+3+...+2019)+1
Aplici formula
1+3+...+2n-1=n²
2n-1=2019
2n=2020
n=1010=>
-(1+3+5+...+2019)= -1010²
S2=1-1010²
S=s1+S2=1010*1011-1010²+1=
1010(1011-1010)+1=
1010*1+1=1011
Explicație pas cu pas: