Ta có $ (x-y)^2\geq 0\Rightarrow x^2+y^2 \geq 2xy\Rightarrow (x^2+y^2) \geq (x+y)^2\Rightarrow \frac{x^2+y^2}{x^2+2xy+y^2} \geq \frac{1}{2}$
Ta có $ (x-y)^2\geq 0\Rightarrow x^2+y^2 \geq 2xy\Rightarrow 2(x^2+y^2) \geq (x+y)^2\Rightarrow \frac{x^2+y^2}{x^2+2xy+y^2} \geq \frac{1}{2}$
Ta có $ (x-y)^2\geq 0\Rightarrow x^2+y^2 \geq 2xy\Rightarrow (x^2+y^2) \geq (x+y)^2\Rightarrow \frac{x^2+y^2}{x^2+2xy+y^2} \geq \frac{1}{2}$