[tex]\displaystyle\bf\\2)\\\\\frac{x-5\sqrt{x} }{x-25} =\frac{(\sqrt{x} )^{2} -5\sqrt{x} }{(\sqrt{x} )^{2} -5^{2} } =\frac{\sqrt{x} \cdot(\sqrt{x} -5)}{(\sqrt{x} -5)\cdot(\sqrt{x} +5)} =\frac{\sqrt{x} }{\sqrt{x}+5 } \\\\\\4)\\\\\frac{\sqrt{15} -5}{3-\sqrt{15} } =\frac{\sqrt{5}\cdot \sqrt{3} -(\sqrt{5} )^{2} }{(\sqrt{3} )^{2} -\sqrt{3} \cdot \sqrt{5} } =\frac{\sqrt{5} \cdot(\sqrt{3} -\sqrt{5}) }{\sqrt{3} \cdot(\sqrt{3}-\sqrt{5} ) } =\frac{\sqrt{5} }{\sqrt{3} }[/tex]