Question

已知x^2+4x-1=0求2x^4+8x^3-4x^2-8x+4的值

Answer 1

2x^4+8x^3-4x^2-8x+4

=2x^4+8x^3-2x^2-2x^2-8x+4

=2x^2(x^2+4x-1)-2x^2-8x+4

=2x^2(x^2+4x-1)-2(x^2+4x-1)+2 [x^2+4x-1=0]

=2