Explicație pas cu pas:
a) 2+4+6+…+88=2(1+2+3+...+44)=2(1+44)*44:2=45*44=1980
b)3+6+9+…+333=3(1+2+3+...+111)=3(1+111)*111:2=112*333:2=56*333=18648
c) 5+10+15+…+2020=5(1+2+3+...+404)=5(1+404)*404:2=5*405*202=409050
d) 8+16+24+…+800=8(1+2+3+...+100)=8(1+100)*100:2=808*50=40400