$\frac {1}{3} \div a=\frac {1}{3} ×\frac {1}{a}=\frac {1}{3\ \mathrm {a}}$
$\frac {1}{a} \div 3=\frac {1}{a} ×\frac {1}{3}=\frac {1}{3\ \mathrm {a}}$
发现:$\frac {1}{3} \div a=\frac {1}{a} \div 3$
将$ a=2 $代入检验得,
$\frac {1}{3} \div a=\frac {1}{3} \div 2=\frac {1}{6}$,$\frac {1}{a} \div 3=\frac {1}{2} \div 3=\frac {1}{6}$。