Câu 4d:
$x\le y\to x-y\le 0$
$\sqrt{(\sqrt{x}-\sqrt{y})^2(\sqrt{x}+\sqrt{y})^2}$
$=\sqrt{[ (\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})]^2}$
$=\sqrt{(x-y)^2}$
$=|x-y|$
Với $x-y=0$: $|x-y|=0$
Với $x-y<0$: $|x-y|=y-x$
Câu 5:
a, $x^2-11=x^2-(\sqrt{11})^2=(x-\sqrt{11})(x+\sqrt{11})$
b, $x^2-2\sqrt{2}x+2=x^2-2\sqrt2.x+(\sqrt2)^2=(x-\sqrt2)^2$
c, $x-5=(\sqrt{x})^2-(\sqrt5)^2=(\sqrt{x}-\sqrt5)(\sqrt{x}+\sqrt5)$
d, $5-7x^2=(\sqrt5)^2-(x\sqrt7)^2=(\sqrt5-x\sqrt7)(\sqrt5+x\sqrt7)$
