January 23, 2020

Chapters

Roots are those values which when substituted in a polynomial, evaluate the whole polynomial equal to zero. In other words, we can say that the root set equal to zero. A polynomial can have multiple roots which means that many numbers can evaluate a polynomial equal to zero. In this article, we will discuss how to find the roots of a polynomial.

## Remainder and Factor Theorem

In mathematics, the remainder and factor theorem are related to the roots or zeros of the polynomial. The reasoning for the factor and the remainder theorem is the same. The remainder theorem is explained below:

"The remainder theorem says that if you divide a polynomial P(y) by its factor x - b, then you will get a zero remainder."

The factor theorem is explained below:

- If P(z) is a polynomial of degree greater than 1 and b is any real number, then is a factor of the polynomial P(z), if and only if .
- , if and only if is the factor of the polynomial P(z). This value is the root or zero of the polynomial P(z).

### Roots of a Polynomial

These are the values that nullify the polynomial.

For example, if x - n is a factor of polynomial P(x), then x = n is the root or zero of the polynomial because when we will substitute the variables in the polynomial by the number n, then we will get 0. A polynomial can have more than one root.

## Properties of the Roots and Factors of a Polynomial

- The
**zeros or roots**are divisors of the independent term of the polynomial. - For each
**root**type**x = a**corresponds to it by a binomial of the type**(x − a)**. - A polynomial can be expressed in factors by writing it as a product of all the
**binomials**of type**(x − a)**, which will correspond to the**roots**,**x = a**. - The sum of the exponents of the binomial must be equal to the degree of the polynomial.
- All polynomials that do not have an independent term accept x = 0 as a root.
- A polynomial is called irreducible or prime when it cannot be decomposed into factors.

## Find the Roots and Factor the Following Polynomial

**Example 1**

Find the roots or zeros of the following polynomial:

**Solution**

We will find the roots or zeros of the above polynomial by factoring the polynomial. We know how to factor a quadratic function. Because the above polynomial is of the form , hence it is a quadratic equation.

Factor the above quadratic function by following the procedure below:

- First, multiply the constant with the first term to get .
- Now, expand the middle term in such a way that it adds up to the midterm and multiplies together to get .
- The first and third term will remain the same.

Now, set the factors equal to zero to find the roots of the polynomial like this:

Either or . Hence, or . This means that -1 and -6 are the roots or zeros of the polynomial. It means that when -1 and -6 will be substituted in the original polynomial equation, we will get 0.

**Example 2**

**Solution**

- First, multiply the constant with the first term to get .
- Now, expand the middle term in such a way that it adds up to the midterm and multiplies together to get .
- The first and third term will remain the same.

Now, pair first and second, and third and fourth terms and find common factors between them:

Set the above factors equal to zero:

Either, or . Hence, or . It means that 2 and -8 are the roots or zeros of the polynomial, i.e. if they are substitute in the original polynomial equation, we will get the answer 0.

**Example 3**

**Solution**

- First, multiply the constant with the first term to get .
- Now, expand the middle term in such a way that it adds up to the midterm and multiplies together to get .
- The first and third term will remain the same.

It means that the factor of the polynomial is . To find the root set . Hence, . This is an example of **double root**.

When we will substitute in the original polynomial function, the resultant answer will be zero.

**Example 4**

**Solution**

- First, multiply the constant with the first term to get .
- The first and third term will remain the same.

Pair the first and second, and third and fourth terms to find the common factors between them:

Set the factored form of the equation equal to zero:

Either, or . Hence, or .

3 and -7 are the roots or zeros of the polynomial, which means if they are substituted in the above polynomial, we will get zero.

**Example 5**

**Solution**

- First, multiply the constant with the first term to get .
- Now, expand the middle term in such a way that it adds up to the middle term and multiplies together to get .
- The first and third term will remain the same.

Set the factors equal to zero like this:

Either or . Hence, x = 3 or x = -4. It means that 3 and -4 are the roots or zeros of the polynomial function. If we will substitute 3 or -4 in the original polynomial equation, we will get the answer 0.

Find the best Maths tutors on Superprof.

**Example 6**

Given is the root of the polynomial , find the other two factors.

**Solution**

This example is different from the rest of the examples, as in this question we are given the root of the polynomial and we are asked to find the other two zeros. It also differs from the rest of the examples because the highest degree of the polynomial is 3 and there are four terms, hence we cannot find the factors using the same process as used in the previous examples.

We know that if is the root of the polynomial, then the factor of the polynomial will be . The above polynomial should be completely divisible by , if it is the factor of the polynomial. The complete divisibility means that the remainder should be 0. To find the other two factors, divide by by long division method:

We know that the polynomial P(x) is equal to the product of its divisor and the quotient. Hence, the polynomial is equal to the product of the factor and the quotient . We can write it mathematically as:

Factor the quotient using the following procedure:

- First, multiply the constant with the first term to get .
- Now, expand the middle term in such a way that it adds up to the middle term and multiplies together to get .
- The first and third term will remain the same.

=

Pair the first and second, and third and fourth terms together and find the common factors between them:

Set the factored form equal to zero:

Either or . Hence, or . It means that -2 and -1 are the zeros or roots of the quotient.

Therefore, there are 3 roots or zeros of the polynomial which are -3, -1 and -2.