[tex]x=\dfrac{3}{\sqrt{7} +\sqrt{5} } -\dfrac{3}{\sqrt{7} -\sqrt{5} }\\\\korzystam~~ze~~wzoru~~skroconego~~mnozenia:~~x^{2} -y^{2} =(x-y)(x+y)\\\\x=\dfrac{3\cdot (\sqrt{7} -\sqrt{5}) -3\cdot (\sqrt{7} +\sqrt{5}) }{(\sqrt{7} +\sqrt{5})\cdot (\sqrt{7} -\sqrt{5}) } \\\\\\\\x=\dfrac{3\sqrt{7} -3\sqrt{5}-3\sqrt{7} -3\sqrt{5} }{(\sqrt{7} )^{2} -(\sqrt{5} )^{2} } \\\\\\x=\dfrac{-6\sqrt{5} }{7-5} \\\\\\x=\dfrac{-6\sqrt{5} }{2} \\\\\\x=-3\sqrt{5} \\\\\\x^{-2} =?~~~~\land ~~~~x=-3\sqrt{5} \\\\[/tex]
[tex](-3\sqrt{5}) ^{-2} =(-\dfrac{1}{3\sqrt{5} } )^{2} =(-\dfrac{1}{3\sqrt{5} })\cdot (-\dfrac{1}{3\sqrt{5} })=\dfrac{1}{9\cdot 5} =\dfrac{1}{45}[/tex]
