a,
+) $(\sqrt{28}-2\sqrt{3}+\sqrt{3})·\sqrt{7}+\sqrt{84}$
$=(2\sqrt{7}-2\sqrt{3}+\sqrt{3})·\sqrt{7}+2\sqrt{3}·\sqrt{7}$
$=(2\sqrt{7}-2\sqrt{3}+\sqrt{3}+2\sqrt{3})·\sqrt{7}$
$=(2\sqrt{7}+\sqrt{3})·\sqrt{7}$
$14+\sqrt{21}$
+) $\sqrt{20}-\sqrt{45}+3\sqrt{18}+\sqrt{72}$
$=2\sqrt{5}-3\sqrt{5}+9\sqrt{2}+6\sqrt{2}$
$=15\sqrt{2}-\sqrt{5}$
b, $\frac{1}{2+\sqrt{3}}+\frac{1}{2-\sqrt{3}}$
$=\frac{2-\sqrt{3}+2+\sqrt{3}}{(2+\sqrt{3})(2-\sqrt{3})}$
$=\frac{4}{4-3}=4$
