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$1) \sqrt{2}( \sqrt{18} +4 \sqrt{2} )= \sqrt{2}( 3\sqrt{2} +4 \sqrt{2} )= \sqrt{2}*7 \sqrt{2}=7*2=14 \\ \\ 2) \frac{ \sqrt{12}- \sqrt{21} }{ \sqrt{3} } =\frac{ 2\sqrt{3}- \sqrt{21} }{ \sqrt{3} }=\frac{ (2\sqrt{3}- \sqrt{21}) \sqrt{3} }{3 }= \frac{6- \sqrt{63} }{3}= \frac{6-3 \sqrt{7} }{3}= \frac{3(2- \sqrt{7} )}{3}= \\ =2- \sqrt{7}$

$3)(2 \sqrt{5}- \sqrt{2} )( \sqrt{15}-5 \sqrt{6} )=2 \sqrt{75}-10 \sqrt{30}- \sqrt{30 }+5\sqrt{12} = \\ = 10\sqrt{3} -11 \sqrt{30}+10 \sqrt{3}=20 \sqrt{3}-11 \sqrt{30}$