左边=(2x²-8xy+8y²)+(x²-4x+4)+(y²+6y+9)=2(x-2y)²+(x-2)²+(y+3)²≥0当x-2y=0,x-2=0,y+3=0取等号但这三个不能同时成立所以等号取不到所以3x²-8xy+9y²-4x+6y+13>0...