a: \(\sqrt{18}-2\cdot\sqrt{50}+3\cdot\sqrt8\)
\(=3\sqrt2-2\cdot5\sqrt2+3\cdot2\sqrt2\)
\(=3\sqrt2+6\sqrt2-10\sqrt2=-\sqrt2\)
b: \(\left(\sqrt7-\sqrt3\right)^2+\sqrt{84}\)
\(=7+3-2\cdot\sqrt{7\cdot3}+2\sqrt{21}\)
\(=10-2\sqrt{21}+2\sqrt{21}=10\)
c: \(\left(\frac{6-2\sqrt2}{3-\sqrt2}-\frac{5}{\sqrt5}\right):\frac{1}{2+\sqrt5}\)
\(=\left(\frac{2\left(3-\sqrt2\right)}{3-\sqrt2}-\sqrt5\right)\left(2+\sqrt5\right)\)
\(=\left(2-\sqrt5\right)\left(2+\sqrt5\right)=4-5=-1\)
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