How to Find Real Zeros of Polynomials

In the world of mathematics, real zeros hold a position of prominence, especially in the context of polynomial functions. Grasping the intricacies of real zeros allows us to delve into the behavior and characteristics of functions. Let's embark on a journey to comprehend real zeros comprehensively.

How to Find Real Zeros of Polynomials

Step-by-step Guide to Find Real Zeros

Here is a step-by-step guide to finding real zeros:

Step 1: Foundational Concepts

  1. Polynomials: Begin by understanding a polynomial. It’s an algebraic expression involving a sum of powers in one or more indeterminates multiplied by coefficients.
  2. Degree of a Polynomial: The highest power of the variable in the polynomial. For instance, in \(3x^3−2x^2+5x−7\), the degree is \(3\).

Step 2: Zero, Root, or \(x\)-Intercept?

  1. Terminology Clarification: Recognize that ‘zero’, ‘root’, and ‘\(x\)-intercept’ are often used interchangeably in the context of polynomial functions. They all denote values of \(x\) for which the polynomial evaluates to zero.

Step 3: Factoring

  1. The Power of Factors: Understand that if \(f(a)=0\) for some polynomial \(f(x)\), then \(x−a\) is a factor of \(f(x)\).
  2. Factoring Techniques: Familiarize yourself with various factoring methods like grouping, the quadratic formula, synthetic division, and long division.

Step 4: The Fundamental Theorem of Algebra

  1. The Core Idea: Every non-constant polynomial function has at least one complex zero.
  2. Implication for Real Zeros: If the coefficients of a polynomial are real and there’s a complex zero \(a+bi\) (where \(b≠0\)), then its conjugate \(a−bi\) is also a zero.

Step 5: The Graphical Insight

  1. Visualization: Plotting a polynomial gives a visual representation of where the function crosses or touches the \(x\)-axis. These points are the real zeros.
  2. Multiplicity Matters: Notice how the graph behaves at each \(x\)-intercept. If it touches and bounces off, the zero has an even multiplicity. If it crosses the axis, it has an odd multiplicity.

Step 6: Rational Root Theorem

  1. Rational Candidate Zeros: For a polynomial with integer coefficients, any rational zero, when expressed in its lowest terms \(\frac{p}{q}\), will have \(p\) as a factor of the trailing constant and \(q\) as a factor of the leading coefficient.
  2. Testing Candidates: Use synthetic division or direct substitution to test potential rational zeros.

Step 7: Descartes’ Rule of Signs

  1. Predicting Zeros: Understand that by counting the number of sign changes in a polynomial, you can predict the possible number of positive real zeros. By substituting \(x\) with \(−x\) and counting the sign changes, you can predict the possible number of negative real zeros.

Step 8: Real Zeros and Derivatives

  1. Link with Calculus: The derivative of a function can indicate where potential zeros or roots might lie. Local maxima and minima, given by the zeros of the derivative, can hint at intervals to investigate for real zeros of the function.

Step 9: Complex Zeros

  1. Understanding their Presence: Know that not all polynomials will have real zeros. Some might only have complex zeros. For instance, \(x^2+1=0\) has no real solution but has complex zeros.

Step 10: Applications of Real Zeros

  1. Problem Solving: Recognize that the knowledge of real zeros is not just theoretical. It’s pivotal in sciences and engineering, especially in topics like signal processing, systems stability, and optimization problems.

Final Words:

Real zeros are the building blocks in understanding polynomial functions, offering insight into the function’s behavior, structure, and graphical representation. Mastering the intricacies of real zeros equips one with an invaluable tool in the diverse realms of algebra, calculus, and beyond. A comprehensive grasp of these zeros bridges the gap between theoretical mathematics and its multifaceted applications in the real world.


Example 1:

Find the zeros of \(f(x)=x^2−9\).


This is a difference of squares, akin to the previous example. The factored form would be:


Given this factorization, the zeros of the polynomial are the values of \(x\) that make \(f(x)=0\). Therefore, by setting each factor to zero, we get:

  1. \(x−3=0\) which gives \(x=3\)
  2. \(x+3=0\) which gives \(x=−3\)

Hence, the zeros of \(f(x)=x^2−9\) are \(x=3\) and \(x=−3\).

Example 2:

Find the real zeros of \(f(x)=x^3−x^2−4x+4\).


To begin, look for factors of the constant term, \(4\), which are \(±1, ±2\), and \(±4\). These are our potential rational zeros according to the Rational Root Theorem.

Using synthetic division or direct substitution, we can test these potential zeros:

For \(x=1\):

Therefore, \(x=1\) is a zero, and \(x−1\) is a factor.

Now, we need to perform polynomial division to determine the quotient when dividing \(f(x)\) by \(x−1\).

Using synthetic division or long division, we find:


Now, \(x^2−4\) is a difference of squares, so it can be further factored as:


Putting it all together, our factored polynomial becomes:


Given this factorization, the zeros of \(f(x)\) are:

  1. \(x=1\)
  2. \(x=2\)
  3. \(x=−2\)

Hence, the real zeros of \(f(x)=x^3−x^2−4x+4\) are \(x=1,2\), and \(−2\).

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