Giải thích các bước giải:
$a)\dfrac{3}{\sqrt{3}-1}-\dfrac{3}{\sqrt{3}+1}$
$=\dfrac{3(\sqrt{3}+1)}{(\sqrt{3}+1)(\sqrt{3}-1)}-\dfrac{3(\sqrt{3}-1)}{(\sqrt{3}-1)(\sqrt{3}+1)}$
$=\dfrac{3(\sqrt{3}+1)}{3-1}-\dfrac{3(\sqrt{3}-1)}{3-1}$
$=\dfrac{3(\sqrt{3}+1)}{2}-\dfrac{3(\sqrt{3}-1)}{2}$
$=\dfrac{3\sqrt{3}+3-3\sqrt{3}+3}{2}$
$=\dfrac{6}{2}$
$=3$
$b)\sqrt{4-\sqrt{7}}.\sqrt{4+\sqrt{7}}$
$=\sqrt{(4-\sqrt{7})(4+\sqrt{7})}$
$=\sqrt{16-7}$
$=\sqrt{9}$
$=3$
Học tốt!!!
