Vieta's Formulas - GeeksforGeeks

Vieta’s Formulas

Difficulty Level : Medium
Last Updated : 04 Apr, 2019

Vieta’s formula relates the coefficients of polynomial to the sum and product of their roots, as well as the products of the roots taken in groups. Vieta’s formula describes the relationship of the roots of a polynomial with its coefficients. Consider the following example to find a polynomial with given roots. (Only discuss real-valued polynomials, i.e. the coefficients of polynomials are real numbers). Let’s take a quadratic polynomial. Given two real roots $r_{1}$ and $r_{2}$ , then find a polynomial.

Consider the polynomial $a_{2}x^{2} + a_{1}x + a_{0}$ . Given the roots, we can also write it as

$k(x - r_{1})(x - r_{2})$ .

Since both equation represents the same polynomial, so equate both polynomial

$a_{2}x^{2} + a_{1}x + a_{0} = k(x - r_{1})(x - r_{2})$

Simplifying the above equation, we get

$a_{2}x^{2} + a_{1}x + a_{0} = kx^{2}-k(r_{1} + r_{2})x + k(r_{1}r_{2})$

Comparing the coefficients of both sides, we get

For $x^{2}$ , $a_{2} = k$ ,

For $x$ , $a_{1} = -k(r_{1} + r_{2})$ ,

For constant term, $a_{0} = kr_{1}r_{2}$ ,

Which gives,

$a_{2} = k$ ,

$\frac{a_{1}}{a_{2}} = -(r_{1} + r_{2})........(1)$

$\frac{a_{0}}{a_{2}} = r_{1}r_{2}..................(2)$

Equation (1) and (2) are known as Vieta’s Formulas for a second degree polynomial.
In general, for an $nth$ degree polynomial, there are n different Vieta’s Formulas. They can be written in a condensed form as

For $0 \leq k \leq n$

$\sum_{1 \leq i_{1} < i_{2} < ... < i_{k} \leq n} (r_{i_{1}}r_{i_{2}} ... r_{i_{k}}) = (-1)^{k}\frac{a_{n-k}}{a_{n}}.$

The following examples illustrate the use of Vieta’s formula to solve a problem.

Examples:

Input : n = 2
        roots = {-3, 2}
Output : Polynomial coefficients: 1, 1, -5

Input : n = 4
        roots = {-1, 2, -3, 7}
Output : Polynomial coefficients: 1, -5, -19, 29, 42

C++

// C++ program to implement vieta formula  
// to calculate polynomial coefficients. 
#include <bits/stdc++.h> 
using namespace std; 
  
// Function to calculate polynomial  
// coefficients. 
void vietaFormula(int roots[], int n) 
{ 
    // Declare an array for 
    // polynomial coefficient. 
    int coeff[n + 1]; 
  
    // Set all coefficients as zero initially 
    memset(coeff, 0, sizeof(coeff)); 
      
    // Set highest order coefficient as 1 
    coeff[n] = 1; 
  
    for (int i = 1; i <= n; i++) { 
        for (int j = n - i - 1; j < n; j++) { 
            coeff[j] = coeff[j] + (-1) *  
                roots[i - 1] * coeff[j + 1]; 
        } 
    } 
  
    cout << "Polynomial Coefficients: "; 
    for (int i = n; i >= 0; i--) { 
        cout << coeff[i] << " "; 
    } 
} 
  
// Driver code 
int main() 
{ 
    // Degree of required polynomial 
    int n = 4; 
      
    // Initialise an array by 
    // root of polynomial 
    int roots[] = { -1, 2, -3, 7 }; 
      
    // Function call 
    vietaFormula(roots, n); 
      
    return 0; 
} 

Java

// Java program to implement vieta formula  
// to calculate polynomial coefficients. 
import java.util.Arrays; 
  
class GFG 
{ 
  
// Function to calculate polynomial  
// coefficients. 
static void vietaFormula(int roots[], int n) 
{ 
    // Declare an array for 
    // polynomial coefficient. 
    int coeff[] = new int[++n + 1]; 
    Arrays.fill(coeff, 0); 
      
    // Set highest order coefficient as 1 
    coeff[n] = 1; 
  
    for (int i = 1; i <n; i++) 
    { 
        for (int j = n - i - 1; j < n; j++) 
        { 
            coeff[j] = coeff[j] + (-1) *  
                roots[i - 1] * coeff[j + 1]; 
        } 
    } 
  
    System.out.print("Polynomial Coefficients: "); 
    for (int i = n; i > 0; i--)  
    { 
        System.out.print(coeff[i] + " "); 
    } 
} 
  
// Driver code 
public static void main(String[] args) 
{ 
    // Degree of required polynomial 
    int n = 4; 
      
    // Initialise an array by 
    // root of polynomial 
    int roots[] = { -1, 2, -3, 7 }; 
      
    // Function call 
    vietaFormula(roots, n); 
    } 
} 
  
/* This code contributed by PrinciRaj1992 */

Python3

# Python3 program to implement  
# Vieta's formula to calculate 
# polynomial coefficients. 
def vietaFormula(roots, n): 
      
    # Declare an array for 
    # polynomial coefficient. 
    coeff = [0] * (n + 1) 
      
    # Set Highest Order  
    # Coefficient as 1 
    coeff[n] = 1
    for i in range(1, n + 1): 
        for j in range(n - i - 1, n): 
            coeff[j] += ((-1) * roots[i - 1] *
                                coeff[j + 1]) 
      
    # Reverse Array  
    coeff = coeff[::-1]  
      
    print("Polynomial Coefficients : ", end = "") 
      
    # Print Coefficients 
    for i in coeff:  
        print(i, end = " ") 
    print() 
  
# Driver Code 
if __name__ == "__main__": 
      
    # Degree of Polynomial 
    n = 4
      
    # Initialise an array by 
    # root of polynomial 
    roots = [-1, 2, -3, 7]  
      
    # Function call 
    vietaFormula(roots, n) 
      
# This code is contributed 
# by Arihant Joshi 

C#

// C# program to implement vieta formula  
// to calculate polynomial coefficients. 
using System; 
      
class GFG 
{ 
  
// Function to calculate polynomial  
// coefficients. 
static void vietaFormula(int []roots, int n) 
{ 
    // Declare an array for 
    // polynomial coefficient. 
    int []coeff = new int[++n + 1]; 
      
    // Set highest order coefficient as 1 
    coeff[n] = 1; 
  
    for (int i = 1; i <n; i++) 
    { 
        for (int j = n - i - 1; j < n; j++) 
        { 
            coeff[j] = coeff[j] + (-1) *  
                roots[i - 1] * coeff[j + 1]; 
        } 
    } 
  
    Console.Write("Polynomial Coefficients: "); 
    for (int i = n; i > 0; i--)  
    { 
        Console.Write(coeff[i] + " "); 
    } 
} 
  
// Driver code 
public static void Main(String[] args) 
{ 
    // Degree of required polynomial 
    int n = 4; 
      
    // Initialise an array by 
    // root of polynomial 
    int []roots = { -1, 2, -3, 7 }; 
      
    // Function call 
    vietaFormula(roots, n); 
} 
} 
  
// This code has been contributed by 29AjayKumar 

PHP

<?php 
// PHP program to implement vieta formula  
// to calculate polynomial coefficients. 
  
// Function to calculate polynomial  
// coefficients. 
function vietaFormula($roots, $n) 
{ 
    // Declare an array for 
    // polynomial coefficient. 
    $coeff = array_fill(0, $n + 1, 0); 
  
    // Set all coefficients as zero initially 
      
    // Set highest order coefficient as 1 
    $coeff[$n] = 1; 
  
    for ($i = 1; $i <= $n; $i++) 
    { 
        for ($j = $n - $i; $j < $n; $j++) 
        { 
            $coeff[$j] = $coeff[$j] + (-1) *  
                         $roots[$i - 1] *  
                         $coeff[$j + 1]; 
        } 
    } 
  
    echo "polynomial coefficients: "; 
    for ($i = $n; $i >= 0; $i--)  
    { 
        echo $coeff[$i]. " "; 
    } 
} 
  
// Driver code 
  
// Degree of required polynomial 
$n = 4; 
  
// Initialise an array by 
// root of polynomial 
$roots = array(-1, 2, -3, 7); 
  
// Function call 
vietaFormula($roots, $n); 
  
// This code is contributed by mits 
?> 

Output:

Polynomial Coefficients: 1 -5 -19 29 42

Time Complexity : $\mathcal{O}(n^{2})$ .

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