Răspuns:
(21 *1+0 ) + (21*1+1) +(21*1+2) + (21*1+3) +....( 21*1+1999)=
21*1000 +(1+2+3+...+1999)= 21*1000 + 1999*2000:2=
21*1000+1999*1000= 1000*(21+1999)= 1000*2020
