(x+y)²-16(x-y)²
=(x+y)²-[4(x-y)]²
={(x+y)+[4(x-y)]}·{(x+y)-[4(x-y)]}
=（x+y+4x-4y)(x+y-4x+4y)
=-（5x-3y)(3x-5y) (x+y)²-16(x-y)²
=(x+y)²-[4(x-y)]²
={(x+y)+[4(x-y)]}·{(x+y)-[4(x-y)]}
=（x+y+4x-4y)(x+y-4x+4y)
=-（5x-3y)(3x-5y)