a, $ĐKXĐ: x \neq 9;x \geq 0$
$P=\dfrac{x.\sqrt[]x-3}{(\sqrt[]x+1)(\sqrt[]x-3)}-\dfrac{2.(\sqrt[]x-3)^2}{(\sqrt[]x+1)(\sqrt[]x-3)}-\dfrac{(\sqrt[]x+3)(\sqrt[]x+1)}{(\sqrt[]x+1)(\sqrt[]x-3)}$
$=\dfrac{x.\sqrt[]x-3-2.(\sqrt[]x-3)^2-(\sqrt[]x+3)(\sqrt[]x+1)}{\sqrt[]x+1)(\sqrt[]x-3)}$
$=\dfrac{x.\sqrt[]x-3-2.x+12.\sqrt[]x-18-x-4.\sqrt[]x-3}{\sqrt[]x+1)(\sqrt[]x-3)}$
$=\dfrac{x.\sqrt[]x-3x+8\sqrt[]x-24}{(\sqrt[]x+1)(\sqrt[]x-3)}$
$=\dfrac{x(\sqrt[]x-3)+8.(\sqrt[]x-3)}{(\sqrt[]x+1)(\sqrt[]x-3)}$
$=\dfrac{(x+8).(\sqrt[]x-3)}{(\sqrt[]x+1)(\sqrt[]x-3)}$
$=\dfrac{x+8}{\sqrt[]x+1}$
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