$\begin{array}{l} b)a = \sqrt {3 + \sqrt 5 } - \sqrt {3 - \sqrt 5 } \left( {a > 0} \right)\\ \Rightarrow {a^2} = 3 + \sqrt 5 + 3 - \sqrt 5 - 2\sqrt {\left( {3 + \sqrt 5 } \right)\left( {3 - \sqrt 5 } \right)} \\ \Rightarrow {a^2} = 6 - 2.\sqrt {9 - 5} = 6 - 2.2 = 2\\ \Rightarrow a = \sqrt 2 \\ c)b = \sqrt {4 - \sqrt 7 } - \sqrt {4 + \sqrt 7 } \left( {b < 0} \right)\\ \Rightarrow {b^2} = 4 - \sqrt 7 + 4 + \sqrt 7 - 2.\sqrt {\left( {4 - \sqrt 7 } \right)\left( {4 + \sqrt 7 } \right)} \\ \Rightarrow {b^2} = 8 - 2.\sqrt {16 - 7} = 8 - 2.3 = 2\\ \Rightarrow b = - \sqrt 2 \\ d)c = \sqrt {6,5 + \sqrt {12} } + \sqrt {6,5 - \sqrt {12} } \left( {c > 0} \right)\\ \Rightarrow {c^2} = 6,5 + \sqrt {12} + 6,5 - \sqrt {12} + 2.\sqrt {\left( {6,5 + \sqrt {12} } \right)\left( {6,5 - \sqrt {12} } \right)} \\ \Rightarrow {c^2} = 13 + 2.\sqrt {42,25 - 12} = 13 + 2.\sqrt {\dfrac{{121}}{4}} = 13 + 2.\dfrac{{11}}{2} = 24\\ \Rightarrow c = \sqrt {24} = 2\sqrt 6 \end{array}$
