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ap dung bat dang thuc mincopxki ta co : \sqrt{x}4.x^2+\1/x^2+ \sqrt{x}(4.y^2+1/y^2) \geqslant \sqrt{x}(2x+2y)^2+ (1/x+1/y)^2 \geqslant\sqrt{x}(2x+2y)^2 + 16/(x+y)^2 = \sqrt{x}4.(x+y)^2 + 16/(x+y)^2. Ap dung bdt cosi cho 2 so duong la 4.(x+y)^2 va 4/(x+y)^2.sau do ap dung cosi co x^2+1=x^2 + 1/4+1/4+1/4+1/4 .roi lai ap dung cosi them 1 lan nua cho y^2+1/4+1/4+1/4+1/4
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