(x^2+y^2)-(xy+x+y-1) =(1/2)*[(x^2-2xy+y^2)+(x^2-2x+1)+(y^2-2y+1)] =(1/2)*[(x-y)^2+(x-1)^2+(y-1)^2] 因为(x-y)^2≥0,(x-1)^2≥0,(y-1)^2≥0 (三项都取=号,有解x=y=1) 所以 (x^2+y^2)-(xy+x+y-1)≥0 x^2+y^2≥xy...