Đặt \(\frac{a}{2017}=\frac{b}{2019}=\frac{c}{2021}=k\)=> a = 2017k, b = 2019k, c = 2021k, thay vào M ta có:
M = \(\frac{\left(2017k-2019k\right).\left(2019k-2021k\right)}{\left(2017k-2021k\right)^2}=\frac{\left(-2k\right)^2}{\left(-4k\right)^2}=\frac{\left(-2k\right)^2}{2^2.\left(-2k\right)^2}=\frac{1}{4}\)