答
-2a²(1/2ab+b²)-5a(a²b-ab²)
=-a³b-2a²b²-5a³b+5a²b²
=3a²b²-6a³b
第二题
3x(x²-2x-1)-2x²(x-2)
=3x³-6x²-3x-2x³+2x²
=x³-4x²-3x