Đáp án:
$A=10$
Giải thích các bước giải:
$A=\sqrt 5.\bigg{(}\dfrac{1}{\sqrt 5-2}+\dfrac{1}{\sqrt 5+2}\bigg{)}$
$=\sqrt 5.\bigg{[}\dfrac{\sqrt 5+2}{(\sqrt 5-2)(\sqrt 5+2)}+\dfrac{\sqrt 5-2}{(\sqrt 5-2)(\sqrt 5+2)}\bigg{]}$
$=\sqrt 5.\bigg{[}\dfrac{\sqrt 5+2+\sqrt 5-2}{(\sqrt 5-2)(\sqrt 5+2)}\bigg{]}$
$=\sqrt 5.\dfrac{2\sqrt 5}{5-4}$
$=10$.
