3:
a: \(A=2\sqrt{7}-3\sqrt{7}+\sqrt{7}+1-\sqrt{7}-1=-\sqrt{7}\)
\(B=\dfrac{\sqrt{x}-3+\sqrt{x}+3}{x-9}\cdot\dfrac{4\left(\sqrt{x}+3\right)}{\sqrt{x}}\)
\(=\dfrac{2\sqrt{x}}{\sqrt{x}}\cdot\dfrac{4}{\sqrt{x}-3}=\dfrac{8}{\sqrt{x}-3}\)
2:
a: \(M=3\sqrt{3}-2\sqrt{3}-\sqrt{3}+1=1\)
\(N=\dfrac{\sqrt{a}+1}{\sqrt{a}\left(\sqrt{a}-1\right)}\cdot\dfrac{\left(\sqrt{a}-1\right)^2}{\sqrt{a}+1}=\dfrac{\sqrt{a}-1}{\sqrt{a}}\)
b: M=2N
=>\(\dfrac{\sqrt{a}-1}{\sqrt{a}}=\dfrac{1}{2}\)
=>2*căn a-2=căn a
=>căn a=2
=>a=4
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