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bình phương 2 vế pt(1): $4x-2\sqrt{4x^{2}-y}=8x-y\Leftrightarrow 2\sqrt{4x^{2}-y}=y-4x\Rightarrow 4(4x^{2}-y)=16x^{2}-4xy+y^{2}\Leftrightarrow y^{2}-8xy+4y=0\Rightarrow y=0(loai) \vee y-8x+4=0\Leftrightarrow y-6x=2x-4 thay vao pt (2) \sqrt{4x^{2}-39}=3+\sqrt{2x-4}\Rightarrow 4x^{2}-39=9+6\sqrt{2x-4}+2x-4\Leftrightarrow 2x^{2}-x-22-3\sqrt{2x-4}=0\Leftrightarrow 2x^{2}-16x+32+15x-60+6-3\sqrt{2x-4}=0\Leftrightarrow 2(x-4)^{2}+15(x-4)-6.\frac{x-4}{\sqrt{2x-4}+2}=0\Rightarrow x=4 \vee 2x+7-\frac{6}{\sqrt{2x-4}+2}\Leftrightarrow 2x+4+\frac{3\sqrt{2x-4}+6-6}{\sqrt{2x-4}+2}=0\Leftrightarrow 2x+4+\frac{3\sqrt{2x-4}}{\sqrt{2x-4}+2}=0 vo ly do x\geq 2. vay voi x=4, y=28 thi thoa DK$
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