（mx²+my²）²-4m²x²y²
=m²(x²+y²)²-4m²x²y²
=m²x^4+2m²x²y²+m²y^4-4m²x²y²
=mx^4-2m²x²y²+m²y^4
=m²(x^4-2x²y²+y^4)
=m²(x²-y²)²
=m²(x+y)²(x-y)²