Cách giải:
$ĐKXĐ:x \geq 0,x \neq 1$
$a,P=(2+\dfrac{a+\sqrt{a}}{\sqrt{a}+1}).(2-\dfrac{a-\sqrt{a}}{\sqrt{a}-1})$
$\to P=(2+\sqrt{a})(2-\sqrt{a})$
$\to P=4-a$
$b,P=\sqrt{\dfrac{\sqrt{2}-1}{\sqrt{2}+1}}$
$\to 4-a=\sqrt{(\sqrt{2}-1)^2}$
$\to 4-a=|\sqrt{2}-1|$
$\to 4-a=\sqrt{2}-1$
$\to a=5-\sqrt{2}$