x2-y2-2x+8y+m=(x²-2x+1)-(y²-8y+16)+m-1+16所以m-1+16=0m=-15 x2-y2-2x+8y-15=(x²-2x+1)-(y²-8y+16)=（x-1）²-(y-4)²=(x-1+y-4)(x-1-y+4)=(x+y-5)(x-y+3)