2+4+6+...........+100= (2+100)·50:2 = 102·25 = 2550
100:2 = 50 termeni
3+6+9+12+..........+2019= (3+2019)·673:2 = 1011·673 = 680403
2019:3 = 673 termeni
4+8+12+..........+2020= (4+2020)·505:2 = 1012·505 = 511060
2020:4 = 505 termeni
7+14+21+...........+2023= (7+2023)·289:2 = 1015·289 = 293335
2023:7 = 289 termeni