[tex]\bf \cfrac{1}{x-10}\div\cfrac{9x}{x^2-9x-10}\implies \cfrac{1}{x-10}\cdot \cfrac{x^2-9x-10}{9x} \\\\\\ \cfrac{1}{\begin{matrix} x-10 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix} }\cdot \cfrac{\begin{matrix} (x-10) \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}~~(x+1)}{9x}\implies \cfrac{x+1}{9x}[/tex]
