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<=> [tex]P=\frac{\sum 4x}{\sum 4\sqrt{y+z-4}}\geq \sum \frac{4x}{y+z+4-4}[/tex] =[tex]4(\sum \frac{x}{y+z})[/tex]
Theo BĐT Nesbit =>
[tex]\sum \frac{x}{y+z}\geq \frac{3}{2}[/tex]
=> [tex]P\geq 4.\frac{3}{2}=6[/tex]
Dấu''='' xảy ra [tex]<=> x=y=z=4[/tex]
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