If $x + \sqrt {({x^2} + 1)} = a,$ then $x =$
${1 \over 2}(a + 1/a)$
${1 \over 2}(a - 1/a)$
$(a + {a^{ - 1}})$
None of these
If ${\left( {{2 \over 3}} \right)^{x + 2}} = {\left( {{3 \over 2}} \right)^{2 - 2x}},$then $x =$
${{\sqrt {(5/2)} + \sqrt {(7 - 3\sqrt 5 )} } \over {\sqrt {(7/2)} + \sqrt {(16 - 5\sqrt 7 )} }}=$
The value of ${{15} \over {\sqrt {10} + \sqrt {20} + \sqrt {40} - \sqrt 5 - \sqrt {80} }}$ is
If $x = {2^{1/3}} - {2^{ - 1/3}},$ then $2{x^3} + 6x = $
${a^{m{{\log }_a}n}} = $