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(x^2+x+1)(x^2-x+1)
여기서 X^+1 = A라 놓으면
(A+x)(A-x) = A^2-x^2
x^4+2x^2+1-x^2
=x^4+x^2+1
(x-1)(x+1)(x+3)(x+5)
=(x-1)(x+5)(x+1)(x+3)
=(x^2+4x-5)(x^2+4x+3)
여기서 x^2+4x=A라 놓으면
(A-5)(A+3) = A^2-2A-15
=(x^2+4x)^2-2(x^2+4)-15
=X^4+8x^3+16x^2-2x^2-8x-15
= X^4+8x^3+14x^2-8x-15
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