应该是(a+2b)²(2a+b)²
=[(a+2b)(2a+b)]²
=(2a²+4ab+2b²)²
=4a⁴+16a²b²+4b⁴+16a³b+8a²b²+16ab³
=4a⁴+24a²b²+4b⁴+16a³b+16ab³