Calculate.

a) [[$ \sqrt {4} \cdot \sqrt {16} = $]]

b) [[$ \sqrt {4 \cdot 16} = $]]

c) [[$ \displaystyle\frac {\sqrt {81}} {\sqrt {9}} = $]]

d) [[$ \sqrt {\displaystyle\frac {81} {9}} = $]]

e) [[$ \sqrt {9^2} = $]]

f) [[$ (\sqrt {9})^2 = $]]