$$\\({a \over c} + {b \over d} = {a·d + b·c\over c·d} )\\$$

$${{\large a \over \large b}\over c} = {a \over b·c}$$

$${{\large a \over \large b}\over { {\large c \over \large d}}} = {a·d \over b·c}$$

$${{1 \over { {\large a \over \large b}}}} = \left({a \over b}\right)^{-1} = {b \over a}$$