计算（a-2)(a^2+2a+4)= (2x-y)(4x^2+2xy+y^2)= 乘法公式：

（a-2)(a^2+2a+4)
=a(a²+2a+4)-2(a²+2a+4)
=a³+2a²+4a-2a²-4a-8
=a³-8
(2x-y)(4x^2+2xy+y^2)
=2x(4x²+2xy+y²)-y(4x²+2xy+y²)
=8x³+4x²y+2xy²-4x²y-2xy²-y³
=8x³-y³
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