（a-2b)(a²+2ab+4b²）-a(a-5b)(a+3b),
=a³-8b³-a(a²-2ab-15b²)
=a³-8b³-a³+2a²b+15ab²
=2a²b+15ab²-8b³
=2x(-1)²x2+15x(-1)x2²-8x2³
=4-60-64
=-120
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