先把原式变形为（x-y）的形式,注意变完形后的式子要和原式相等,即：原式=3[-（x-y）]²+2(x-y)+[-(x-y)]-(x-y)² =3(x-y)²+2(x-y)-(x-y)-(x-y)² =2(x-y)²+(x-y)...