$\displaystyle \frac{1}{6}$\displaystyle \frac{1}{6}

$\displaystyle \frac{1}{9}$\displaystyle \frac{1}{9}

$\displaystyle \frac{1}{12}$\displaystyle \frac{1}{12}

$\displaystyle \frac{1}{18}$\displaystyle \frac{1}{18}