4x*(x平方-1/2x-1)
=4x*x²-4x×1/2x-4x*1
=4x³-2x²-4x
a(a-4b)-b(a+2b)
=a²-4ab-ab-2b²
=a²-5ab-2b²
(x+2)(x-4)-x(1-2x)
=x²-2x-8-x+2x²
=3x²-3x-8
6x(x+1)-(2x+3)(3x-1)
=6x²+6x-(6x²+7x-3)
=-x+3