- $1890=2.3^3.5.7$
- $1930=2.5.193$
- $1945=5.389$
- $1975=5^2.79$
- $2012=2^2.503$
$\Rightarrow a=1890.1930.1945.1975.2012=(2.3^3.5.7)(2.5.193)(5.389)(5^2.79)(2^2.503)=(2.2.2^2).3^3.(5.5.5.5^2).7.79.193.389.503=2^4.3^3.5^5.7.79.193.389.503$
a có số ước là : $(4+1)(3+1)(5+1)(1+1)(1+1)(1+1)(1+1)(1+1)=3840$
Vậy Ư(a) có 3840 phần tử