Yahoo Answers is shutting down on May 4th, 2021 (Eastern Time) and beginning April 20th, 2021 (Eastern Time) the Yahoo Answers website will be in read-only mode. There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page.

How to reduce an equation of nth degree to a quadratic?

Hi everyone, I was studying the cyclotomic equation and I came across an example:

x^4 + x^3 + x^2 + x +1 = 0 ; this equation cannot be factorised neither using Ruffini's rule nor replacing an x with t. The book said that it is sufficient to divide all by x^2 and rearrange the terms to obtain --> x^2 + 1/x^2 + x + 1/x + 1 = 0, then let w be equal to x + 1/x, and the equation (I also have verified) becomes w^2 + w -1 = 0.

My question is: does it exist a formula (or a procedure) that allows to reduce an equation with a degree greater than 2 to a quadratic? Procedures other than replace x^4 with t and x^2 with t.

In fact there would be Abel-Ruffini theorem to take into account...which says that there is not a general algebraic solution for quintic equations and higher degrees...

Could you help me please?

1 Answer

Relevance
  • 10 years ago
    Favorite Answer

    Hi,

    it works for every cyclotomic polynomial since the splitting fields of these polynomials over Q have abelian Galois groups (in particular, the groups are solvable). It doesn't work in general - the Abel-Ruffini theorem tells you that.

Still have questions? Get your answers by asking now.