Q. \(x^4 – 10x^3 + 35x^2 – 50x + 25\) factorization.

Answer

Note that
\[
(x^2 – 5x + 5)^2 = x^4 – 10x^3 + 35x^2 – 50x + 25,
\]
so the factorization is
\[
x^4 – 10x^3 + 35x^2 – 50x + 25 = (x^2 – 5x + 5)^2
= \left(x – \tfrac{5+\sqrt{5}}{2}\right)^2 \left(x – \tfrac{5-\sqrt{5}}{2}\right)^2.
\]

Detailed Explanation

  1. Write the polynomial to factor:

    \[x^{4} – 10x^{3} + 35x^{2} – 50x + 25\]

  2. Try to see if it is a perfect square of a quadratic. Assume

    \[\bigl(x^{2} + p x + q\bigr)^{2} = x^{4} – 10x^{3} + 35x^{2} – 50x + 25.\]

    Expand the left-hand side:

    \[
    \bigl(x^{2} + p x + q\bigr)^{2}
    = x^{4} + 2p\,x^{3} + \bigl(p^{2} + 2q\bigr)x^{2} + 2pq\,x + q^{2}.
    \]

  3. Equate coefficients of like powers of x between the expanded form and the given polynomial:

    • Coefficient of x^{3}: \[2p = -10 \implies p = -5.\]
    • Coefficient of x^{2}: \[p^{2} + 2q = 35.\] Substitute p = -5 to get \[25 + 2q = 35 \implies 2q = 10 \implies q = 5.\]
    • Verify the remaining coefficients:

      Coefficient of x: \[2pq = 2(-5)(5) = -50\] matches.

      Constant term: \[q^{2} = 5^{2} = 25\] matches.

  4. Therefore the polynomial is the square of the quadratic found:

    \[\boxed{\bigl(x^{2} – 5x + 5\bigr)^{2}}\]

  5. If desired, factor further into linear factors over the real numbers by finding the roots of x^{2} – 5x + 5. Compute the discriminant:

    \[\Delta = (-5)^{2} – 4\cdot 1\cdot 5 = 25 – 20 = 5.\]

    The roots are

    \[
    x = \frac{5 \pm \sqrt{5}}{2}.
    \]

    Hence the full factorization into linear real factors is

    \[
    \boxed{\biggl(x – \frac{5 + \sqrt{5}}{2}\biggr)^{2}\biggl(x – \frac{5 – \sqrt{5}}{2}\biggr)^{2}}.
    \]

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FAQs

Is the polynomial a perfect square?

Yes. x^4 - 10x^3 + 35x^2 - 50x + 25 = (x^2 - 5x + 5)^2.

How can you spot the perfect-square structure quickly?

Try (x^2 + ax + b)^2 and match coefficients: 2a = -10 gives a = -5, b^2 = 25 gives b = ±5; b = 5 matches 2ab = -50, so (x^2 - 5x + 5)^2.

Does it factor over the rationals into linear factors?

No. x^2 - 5x + 5 has discriminant 5, not a perfect square, so it is irreducible over Q; the quartic is the square of that irreducible quadratic.

What are the roots and their multiplicities?

The quadratic roots are (5 ± √5)/2, and each root has multiplicity 2 (double roots), so two distinct real roots, each repeated twice.

How does it factor over the real numbers?

Over R it splits into linear factors with radicals: (x - (5+√5)/2)^2 (x - (5-√5)/2)^2.

How would you factor it using substitution or symmetry?

How would you factor it using substitution or symmetry?

Can you use derivatives to detect repeated roots?

Yes. If the quartic and its derivative share a common factor, roots repeat. Compute \(\gcd(f, f')\) and find \(x^2 - 5x + 5\) divides both, confirming double roots (multiplicity > 1).

How to express the factorization over the complex numbers?

Same as over the reals since roots are real: (x - (5+√5)/2)^2 (x - (5-√5)/2)^2; no nonreal complex roots occur.
Useful factorization tools are below.
Try them to check solutions.
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