`a) f (x) = (sin x + 1)/(sin x - 1)`
`ĐK:`
`sin x ne 1`
`-> x ne (\pi)/2 + k2pi` `(k in ZZ)`
`-> D = RR \\ {(\pi)/2 + k2pi | k in ZZ}`
`b) f (x) = (2tan x + 2)/(cos x - 1)`
`ĐK:`
\(\left\{ \begin{array}{l}cos x \ne 0\\cos x \ne 1\end{array} \right.\)
`->` \(\left\{ \begin{array}{l}x \ne \dfrac{\pi}{2} + k\pi\\x \ne k2\pi\end{array} \right.\)
`-> D = RR \\ {(\pi)/2 + kpi; k2pi | k in ZZ}`
`c) f (x) = (cot x)/(sin x + 1)`
`ĐK:`
\(\left\{ \begin{array}{l}sin x \ne 0\\sin x \ne -1\end{array} \right.\)
`->` \(\left\{ \begin{array}{l}x \ne k\pi\\x \ne \dfrac{-\pi}{2} + k2\pi\end{array} \right.\)
`-> D = RR \\ {kpi; (-\pi)/2 + k2pi | k in ZZ}`
