∵2xy =（x+y）^2-（x^2+y^2） =（x+y）^2-1 =（x+y+1）（x+y-1）∴2xy/(x+y-1) =x+y+1 ≥-√[2（x^2+y^2）]+1 =1-√2，∴2xy/(x+y-1)的最小值为1-√2.
