Difference between revisions of "Vieta's Formulas"

(Redirected page to Vieta's formulas)
(Tag: New redirect)
 
(92 intermediate revisions by 62 users not shown)
Line 1: Line 1:
== Introduction ==
+
#REDIRECT[[Vieta's formulas]]
 
 
Let <math>P(x)={a_n}x^n+{a_{n-1}}x^{n-1}+\cdots+{a_1}x+a_0</math>,
 
where the coefficient of <math>\displaystyle x^{i}</math> is <math>\displaystyle {a}_i</math>. As a consequence of the [[Fundamental Theorem of Algebra]], we can also write <math>P(x)=a_n(x-r_1)(x-r_2)\cdots(x-r_n)</math>, where <math>\displaystyle {r}_i</math> are the roots of <math>\displaystyle P(x)</math>.  We thus have that
 
 
 
<math> a_nx^n+a_{n-1}x^{n-1}+\cdots+a_1x+a_0 = a_n(x-r_1)(x-r_2)\cdots(x-r_n).</math>
 
 
 
Expanding out the right hand side gives us
 
 
 
<center><math> a_nx^n - a_n(r_1+r_2+\cdots+r_n)x^{n-1} + a_n(r_1r_2 + r_1r_3 + \cdots + r_{n-1}r_n)x^{n-2} + \cdots + (-1)^na_n r_1r_2\cdots r_n.</math></center>
 
 
 
We can see that the coefficient of <math>\displaystyle  x^k </math> will be the <math> \displaystyle k </math>th [[symmetric sum]].  The <math>k</math>th symmetric sum is just the sum of the roots taken <math>k</math> at a time.  For example, the 4th symmetric sum is <math>\displaystyle r_1r_2r_3r_4 + r_1r_2r_3r_5+\cdots+r_{n-3}r_{n-2}r_{n-1}r_n.</math>  Notice that every possible [[combination]] of four roots shows up in this sum.
 
 
 
We now have two different expressions for <math>\displaystyle P(x)</math>.  These ''must'' be equal.  However, the only way for two polynomials to be equal for all values of <math>\displaystyle x</math> is for each of their corresponding coefficients to be equal.  So, starting with the coefficient of <math>\displaystyle  x^n </math>, we see that
 
 
 
<center><math>\displaystyle a_n = a_n</math></center>
 
<center><math> a_{n-1} = -a_n(r_1+r_2+\cdots+r_n)</math></center>
 
<center><math> a_{n-2} = a_n(r_1r_2+r_1r_3+\cdots+r_{n-1}r_n)</math></center>
 
<center><math>\vdots</math></center>
 
<center><math>a_0 = (-1)^n a_n r_1r_2\cdots r_n</math></center>
 
 
 
More commonly, these are written with the roots on one side and the <math>\displaystyle a_i</math> on the other (this can be arrived at by dividing both sides of all the equations by <math>\displaystyle a_n</math>).
 
 
 
If we denote <math>\displaystyle \sigma_k</math> as the <math>\displaystyle k</math>th symmetric sum, then we can write those formulas more compactly as <math>\displaystyle \sigma_k = (-1)^k\cdot \frac{a_{n-k}}{a_n{}}</math>, for <math>\displaystyle 1\le k\le {n}</math>.
 
 
 
== See also ==
 
 
 
* [[Algebra]]
 
* [[Polynomials]]
 
* [[Newton sums]]
 
 
 
== Related Links ==
 
[http://mathworld.wolfram.com/VietasFormulas.html Mathworld's Article]
 

Latest revision as of 13:40, 5 November 2021

Redirect to: