$\displaystyle \begin{array}{{>{\displaystyle}l}} \ \frac{x-1}{\sqrt{5-3x}} \ xác\ định\ khi\ \frac{x-1}{\sqrt{5-3x}} \geqslant 0\ \\ \rightarrow \sqrt{5-3x} >0\ hay\ 5-3x >0\ \\ \rightarrow x< \frac{5}{3} \ \\ Vậy\ với\ x< \frac{5}{3} \ thì\ \frac{x-1}{\sqrt{5-3x}} \ xác\ định\ \ \end{array}$