Cho các số x, y cùng dấu. CM rằng:
a) \(\dfrac{x}{y}+\dfrac{y}{x}\ge2\)
b) \(\left(\dfrac{x^2}{y^2}+\dfrac{y^2}{x^2}\right)-\left(\dfrac{x}{y}+\dfrac{y}{x}\right)\ge0\)
c)\(\left(\dfrac{x^4}{y^4}+\dfrac{y^4}{x^4}\right)-\left(\dfrac{x^2}{y^2}+\dfrac{x^2}{y^2}\right)+\left(\dfrac{x}{y}+\dfrac{y}{x}\right)\ge2\)