x^2+4x=1
2x^4+8x^3-4x^2-8x+1=2x^2(x^2+4x）-4x^2-8x+1
=-2x^2-8x+1
=-2（x^2+4x）+1
=-1
因为 2x^4+8x^3-4x^2-8x+1
=2x^4+8x^3-2x^2-2x^2-8x+1
=2x^2(x^2+4x-1)-2x^2-8x+1
=2x^2(x^2+4x-1)-2x^2-8x+2-1
=2x^2(x^2+4x-1)-2(x^2+4x-1)-1
又因为 x^2+4x-1=0
所以 2x^4+8x^3-4x^2-8x+1
=(2x^2)*0-2*0-1
=-1
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