Đặt A=1/1x2x3 + 1/2x3x4 + 1/3x4x5 + ... + 1/2018x2019x2020
⇒ 2A = 2/1x2x3 + 2/2x3x4 + 2/3x4x5 + ... + 2/2018x2019x2020
⇒ 2A= -1/1x2 + 1/2x3 - 1/2x3 +1/3x4 - 1/3x4 + 1/4x5 + ... -1/2018x2019 + 1/2019x2020
⇒ 2A = -1/1x2 + 1/2019x2020
A = 2039189/4078380