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Here is the problem: x to the power 4 minus 5x squared plus 4.

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Factor a polynomial over the rational numbers.

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Write the polynomial in standard form.

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Apply the Rational Root Theorem. Any rational root p/q has p dividing the constant term 4 and q dividing the leading coefficient 1. Testing these candidates gives the roots negative 2, negative 1, 1, 2.

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Divide by (x plus 2) using synthetic division at x equals negative 2.

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Divide by (x plus 1) using synthetic division at x equals negative 1.

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Divide by (x minus 1) using synthetic division at x equals 1.

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Divide by (x minus 2) using synthetic division at x equals 2.

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Write the complete factorization.

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So the answer is (x minus 2)(x minus 1)(x plus 1)(x plus 2).

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x to the power 4 minus 5x squared plus 4 factors completely over the rationals as (x minus 2)(x minus 1)(x plus 1)(x plus 2).

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Expanding (x minus 2)(x minus 1)(x plus 1)(x plus 2) gives back x to the power 4 minus 5x squared plus 4.