Đáp án:
a) $A=\dfrac{3}{4}$
b) $B=\dfrac{2}{\sqrt x+1}$
Giải thích các bước giải:
ĐKXĐ: $x\ge 0;x\ne 1$
a) $A=\dfrac{\sqrt x+1}{\sqrt x+2}$
Với $x=4$
$⇒A=\dfrac{\sqrt 4+1}{\sqrt 4+2}=\dfrac{2+1}{2+2}=\dfrac{3}{4}$
b) $B=\dfrac{3}{\sqrt x-1}-\dfrac{\sqrt x+5}{x-1}$
$=\dfrac{3(\sqrt x+1)}{(\sqrt x-1)(\sqrt x+1)}-\dfrac{\sqrt x+5}{x-1}$
$=\dfrac{3\sqrt x+3-\sqrt x-5}{(\sqrt x-1)(\sqrt x+1)}$
$=\dfrac{2(\sqrt x-1)}{(\sqrt x-1)(\sqrt x+1)}$
$=\dfrac{2}{\sqrt x+1}$.
