\frac{a}{4b^{2} } = a - \frac{4ab^{2}}{4b^{2} }\geq a - \frac{4ab^2}{4b} = a -ab
Tương tự \frac{b}{4b^{2} }\geq b - ab
Suy ra Q \geq a+b-ab=4ab-2ab =2ab
Mà theo gt: a+b=4ab nên \frac{1}{a} +\frac{1}{b} = 4
suy ra 4 \geq \frac{2}{\sqrt[2]{ab} }
suy ra ab \geq 1/4
Suy ra Q \geq 1/2
Dấu "=" xảy ra khi a=b=1/2