If $a = \sqrt {(21)} - \sqrt {(20)} $ and $b = \sqrt {(18)} - \sqrt {(17),} $ then
$a = b$
$a + b = 0$
$a > b$
$a < b$
If $x = {2^{1/3}} - {2^{ - 1/3}},$ then $2{x^3} + 6x = $
${{\sqrt {6 + 2\sqrt 3 + 2\sqrt 2 + 2\sqrt 6 } - 1} \over {\sqrt {5 + 2\sqrt 6 } }}$
$\root 4 \of {(17 + 12\sqrt 2 )} = $
${{{{[4 + \sqrt {(15)} ]}^{3/2}} + {{[4 - \sqrt {(15)} ]}^{3/2}}} \over {{{[6 + \sqrt {(35)} ]}^{3/2}} - {{[6 - \sqrt {(35)} ]}^{3/2}}}} = $
The value of $\sqrt {[12 - \sqrt {(68 + 48\sqrt 2 )} ]} = $