Tính

a) 1/1x2 + 1/2x3 + 1/3x4 +.... + 1/2020x2021
b) 1/2x4 + 1/4x6 + 1/6x8 +....+ 1/2020x2022
c 1/100 - 1/100x99 - 1/99x98 - .... 1/2x1