ĐKXĐ: x>=0
a: \(C=\frac{1}{\sqrt{x}+1}-\frac{3}{x\cdot\sqrt{x}+1}+\frac{1}{x-\sqrt{x}+1}\)
\(=\frac{x-\sqrt{x}+1-3+\sqrt{x}+1}{\left(\sqrt{x}+1\right)\left(x-\sqrt{x}+1\right)}=\frac{x-1}{\left(\sqrt{x}+1\right)\left(x-\sqrt{x}+1\right)}\)
\(=\frac{\sqrt{x}-1}{x-\sqrt{x}+1}\)
b: C<1
=>C-1<0
=>\(\frac{\sqrt{x}-1-x+\sqrt{x}-1}{x-\sqrt{x}+1}<0\)
=>\(-x+2\sqrt{x}-2<0\)
=>\(x-2\sqrt{x}+2>0\)
=>\(\left(\sqrt{x}-1\right)^2+1>0\) (luôn đúng)
=>x>=0
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