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a) Ta có: \(VT=8-2\sqrt{7}\)
\(=7-2\cdot\sqrt{7}\cdot1+1\)
\(=\left(\sqrt{7}-1\right)^2\)
=VP(đpcm)
b) Ta có: \(VT=\sqrt{8-2\sqrt{7}}-\sqrt{8+2\sqrt{7}}\)
\(=\sqrt{7-2\cdot\sqrt{7}\cdot1+1}-\sqrt{7+2\cdot\sqrt{7}\cdot1+1}\)
\(=\sqrt{\left(\sqrt{7}-1\right)^2}-\sqrt{\left(\sqrt{7}+1\right)^2}\)
\(=\left|\sqrt{7}-1\right|-\left|\sqrt{7}+1\right|\)
\(=\sqrt{7}-1-\left(\sqrt{7}+1\right)\)
\(=\sqrt{7}-1-\sqrt{7}-1=-2=VP\)(đpcm)