9(x-y)²-y²=[3(x-y)]²-y²=[3(x-y)+y][3(x-y)-y]=(3x-2y)(3x-4y) x²+y²-8y+6x+25=0(x²+6x+9)+(y²-8y+16)=0(x+3)²+(y-4)²=0因为(x+3)²≥0 (y-4)²≥0...