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Tính tử riêng cho dễ nhìn
Ta có:$B = \sqrt {7 + \sqrt 5 } + \sqrt {7 - \sqrt 5 } $
$ \to {B^2} = {\left( {\sqrt {7 + \sqrt 5 } + \sqrt {7 - \sqrt 5 } } \right)^2} = 7 + \sqrt 5 + 7 - \sqrt 5 + 2\sqrt {\left( {7 + \sqrt 5 } \right)\left( {7 - \sqrt 5 } \right)} = 14 + 2\sqrt {49 - 5} = 14 + 2\sqrt {44} = 2\left( {7 + 2\sqrt {11} } \right)$
$ \to B = \sqrt 2 .\sqrt {7 + 2\sqrt {11} } $
$ \to A = \dfrac{{\sqrt 2 .\sqrt {7 + 2\sqrt {11} } }}{{\sqrt {7 + 2\sqrt {11} } }} - \sqrt {3 - 2\sqrt 2 } = \sqrt 2 - \sqrt {{{\left( {\sqrt 2 - 1} \right)}^2}} = \sqrt 2 - \sqrt 2 + 1 = 1$
 
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