`(a^2)/x+(b^2)/y>=[(a+b)^2]/(x+y)`
`<=>(a^2y+b^2x)/(xy)>=[(a+b)^2]/(x+y)`
`=>a^2xy+a^2y^2+b^2x^2+b^2xy>=xy(a^2+2ab+b^2)`
`<=>a^2xy+a^2y^2+b^2x^2+b^2xy>=a^2xy+2axby+b^2xy`
`<=>a^2y^2-2aybx+b^2x^2>=0`
`<=>(ay-bx)^2>=0`
dấu = xảy ra khi `ax=by<=>a/y=b/x`
@vương 2k_5