a, Vì \(\hept{\begin{cases}120a⋮12\\36b⋮12\end{cases}\Rightarrow120a+36b⋮12}\)
b, 57 - 56 + 55 = 55(52 - 5 + 1) = 52.21 \(⋮\)21
c, 52003 + 52002 + 52001 = 52001(52+5+1) = 52001.31 \(⋮\)31
d, 1019 + 1018 + 1017 = 1016(103+102+10) = 1016.1110 = 1016.2.555 \(⋮\)555