(x⁴+y⁴)²-4x⁴y⁴=x⁸+2x⁴y⁴+y⁸-4x⁴y⁴=x⁸-2x⁴y⁴+y⁸=(x⁴-y⁴)²=(x²+y²)²(x²-y²)...