$\displaystyle \begin{array}{{>{\displaystyle}l}} 7.\ \frac{2}{x-2} +\frac{3}{x-3} =\frac{3}{x-4}\\ ĐK\ x\neq \{2;3;4\}\\ \Leftrightarrow \frac{5x-12}{x^{2} -5x+6} =\frac{3}{x-4}\\ \Leftrightarrow 5x^{2} -32x+48=3x^{2} -15x+18\\ \Leftrightarrow 2x^{2} -17x+30=0\\ \Leftrightarrow ( x-6)( 2x-5) =0\\ \Leftrightarrow x=6\ ( TM) \ \ or\ x=\frac{5}{2} \ ( TM)\\ Vậy\ S=\left\{6;\frac{5}{2}\right\}\\ 9.\ \frac{7}{x-4} +1=\frac{3}{x^{2} -5x+4} -\frac{1}{x-1}\\ ĐK\ x\neq \{4;1\}\\ \Leftrightarrow \frac{7x-7+x^{2} -5x+4+x-4}{x^{2} -5x+4} =\frac{3}{x^{2} -5x+4}\\ \Leftrightarrow x^{2} +3x-10=0\\ \Leftrightarrow ( x-2)( x+5) =0\\ \Leftrightarrow x=2\ ( TM) \ or\ x=-5\ ( TM)\\ Vâỵ\ S=\{2;-5\} \end{array}$