S = 5 + 52 + 53 + ... + 52020
5S = 52 + 53 + 54 +... + 52021
5S - S = (52 + 53 + 54 + ... + 52021) - (5 + 52 + 53 + ... + 52020)
4S = 52 + 53 + 54 + ... + 52021 - 5 - 52 - 53 - ... 52020
4S = (52 - 52) + (53 - 53) +(54 - 54)+ ... +(52020- 52020) + (52021 - 5)
4S + 5 = 52021 - 5 + 5
4S + 5 = 52021 (đpcm)