解m²(x-y)+n²(y-x)=m²(x-y)-n²(x-y)=(x-y)(m²-n²)=(x-y)(m-n)(m+n)16(x+y)²-25(x-y)²=[4(x+y)]²-[5(x-y)]²=[4(x+y)-5(x-y)][4(x+y)+5(x-y)]=(9y-x)(9x-y)