原式=(a^4-2a²b²+b^4)+c^4+2a²c²-2b²c²-4a²c²
=(a²-b²)²+2c²(a²-b²)+c^4-4a²c²
=(a²-b²+c²)²-4a²c²
=(a²-b²+c²+2ac)(a²-b²+c²-2ac)
=[(a+c)²-b²][(a-c)²-b²]
=(a+b+c)(a-b+c)(a+b-c)(a-b-c)