Proof: If a Polynomial & its Derivative have Same Root

Click For Summary

Discussion Overview

The discussion centers around the relationship between a polynomial and its derivative when they share a common root. Participants explore whether the assertion that if a polynomial \( f(x) \) and its derivative \( f'(x) \) both equal zero at a point \( c \), then \( f(x) \) can be expressed in a specific factored form, is true. The scope includes theoretical exploration and proofs related to polynomial functions and their derivatives.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant proposes that if \( f(c) = f'(c) = 0 \), then \( f(x) \) can be expressed as \( f(x) = (x-c)^m h(x) \), where \( m \) is an integer greater than 1 and \( h(x) \) is a polynomial.
  • Several participants find the assertion trivial, suggesting that it may be straightforward if one knows the appropriate methods to apply.
  • One participant mentions having proved the statement using the factor theorem but expresses interest in finding a more elementary proof.
  • Another participant reiterates the use of the factor theorem, indicating that if \( f(c) = 0 \), then \( f(x) \) can be factored as \( f(x) = (x-c)g(x) \) and discusses the derivative \( f'(x) \) in relation to this factorization.

Areas of Agreement / Disagreement

Participants generally agree that the assertion may be trivial under certain conditions, but there is no consensus on the existence of a more elementary proof or the completeness of the proposed factorization.

Contextual Notes

The discussion does not resolve the assumptions regarding the conditions under which the polynomial and its derivative share a root, nor does it clarify the implications of the factor theorem in this context.

Happiness
Messages
686
Reaction score
30
Given a polynomial ##f(x)##. Suppose there exists a value ##c## such that ##f(c)=f'(c)=0##, where ##f'## denotes the derivative of ##f##. Then ##f(x)=(x-c)^mh(x)##, where ##m## is an integer greater than 1 and ##h(x)## is a polynomial.

Is it true? Could you prove it?

Note: The converse is true and can be proved easily.
 
Physics news on Phys.org
Looks quite trivial to me.
 
mfb said:
Looks quite trivial to me.

It's trivial if we know what to use.

I managed to prove it using factor theorem. But I guessed I wanted to see if there is a more elementary proof when I posted the question. And then I found an elementary proof for factor theorem.
 
Happiness said:
It's trivial if we know what to use.

I managed to prove it using factor theorem. But I guessed I wanted to see if there is a more elementary proof when I posted the question. And then I found an elementary proof for factor theorem.

If ##f(c) = 0## then ##f(x) = (x-c)g(x)## and ##f'(x) = g(x) + (x-c)g'(x) \dots##
 
  • Like
Likes   Reactions: mfb

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 12 ·
Replies
12
Views
3K
  • · Replies 21 ·
Replies
21
Views
3K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K