原式=a³-6a²b+8ab²+4ab²-8b³
=a(a²-6ab+8b²)+4b²(a-2b)
=a(a-2b)(a-4b)+4b²(a-2b)
=(a-2b)(a²-4ab+4b²)
=(a-2b)(a-2b)²
=(a-2b)³