2.4 - Factorisation of Polynomials

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Understanding the Factor Theorem

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Teacher
Teacher

Today, we're going to explore the Factor Theorem. Can anyone tell me what a factor of a polynomial is?

Student 1
Student 1

Is a factor something that divides the polynomial completely without leaving a remainder?

Teacher
Teacher

That's exactly right! If we have a polynomial p(x), then x - a is a factor if p(a) = 0. Remember this as the 'zero leads to factor' rule.

Student 2
Student 2

So if I substitute a number into a polynomial and it equals 0, does that mean that number is a root?

Teacher
Teacher

Correct! This also means that (x-a) is a factor. This connection is essential for the factorisation process.

Student 3
Student 3

What happens if we have a polynomial of degree greater than one?

Teacher
Teacher

Great question! The same principles apply regardless of the degree; we simply look for all roots to factor the polynomial completely.

Student 4
Student 4

Can you give an example of using this theorem?

Teacher
Teacher

Sure! Let's say we have p(x) = x^2 - 3x + 2. If we find p(1) = 0, then (x - 1) is a factor. Can anyone calculate the value of p for x = 2?

Student 1
Student 1

At x = 2, p(2) = 2^2 - 3(2) + 2, which gives 0, so (x - 2) is also a factor!

Teacher
Teacher

Excellent work! Remember, finding roots helps in factorising the polynomial into simpler forms.

Factorisation Techniques Overview

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Teacher
Teacher

Now that we understand the Factor Theorem, let’s explore how to factor quadratics like ax² + bx + c. What do you think is the first step?

Student 1
Student 1

I think we should look for two numbers that add up to b and multiply to ac.

Teacher
Teacher

Exactly! We can use these numbers to split the middle term. For example, in 6x² + 17x + 5, what are our values?

Student 2
Student 2

The product is 30 and the sum is 17. I think it’s 2 and 15!

Teacher
Teacher

Great job! This allows us to write it as 6x² + 2x + 15x + 5. Now, can anyone tell me what to do next?

Student 3
Student 3

We can group the terms and factor them separately, right?

Teacher
Teacher

Right again! Let's practice this technique together with multiple examples to solidify our understanding.

Student 4
Student 4

Can we also use the Factor Theorem in this context?

Teacher
Teacher

Yes! Finding the roots first can simplify our factorisation significantly.

Cubic Polynomials and Advanced Factorisation

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Teacher
Teacher

Let’s move on to cubic polynomials. How do we start factorising a cubic polynomial like x³ - 23x² + 142x - 120?

Student 1
Student 1

We could find one root first, and then factor it step by step.

Teacher
Teacher

Exactly! After discovering a root through the Factor Theorem, we can reduce the cubic to a quadratic to finish factoring.

Student 2
Student 2

How do we find the roots efficiently?

Teacher
Teacher

Good question! We often test simple factors, using the coefficients to guide our trials. Let's try testing "+-1, +-2, +-3" with this polynomial.

Student 3
Student 3

So we can just plug in these values and check p(1) or p(-1) to see if they equal zero?

Teacher
Teacher

Yes! Once we find one, we divide the cubic polynomial by that factor, simplifying it into quadratic form.

Student 4
Student 4

Why do we prefer quadratics for our final factorisation?

Teacher
Teacher

Quadratic polynomials are easier to factor. We can apply the techniques we learned earlier! Let's wrap this session by summarizing the ways we can factor both quadratic and cubic polynomials.

Introduction & Overview

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Quick Overview

This section introduces the concept of factorisation of polynomials and the Factor Theorem, illustrating how to identify polynomial factors based on their roots.

Standard

In this section, we explore the factorisation of polynomials, particularly focusing on the Factor Theorem, which connects a polynomial's zeroes with its factors. Various examples illustrate how to apply these concepts to quadratic and cubic polynomials, highlighting the significance of finding roots.

Detailed

Factorisation of Polynomials

In this section, we delve into the process of factorising polynomials, a crucial aspect of algebra that simplifies understanding polynomial behavior. The Factor Theorem plays a central role, stating that if a polynomial p(x) of degree n has a real number a such that p(a) = 0, then (x - a) is a factor of p(x). Conversely, if (x - a) is a factor of p(x), then p(a) = 0. This theorem arises from the Remainder Theorem, which establishes that dividing a polynomial by (x - a) yields a remainder of zero when a is a root.

The section also demonstrates factorisation techniques for quadratic polynomials, suggesting methods such as splitting the middle term or using the Factor Theorem. For instance, to factor a polynomial like ax² + bx + c, we must express b as the sum of two numbers whose product equals ac.

Examples illustrate these processes, such as checking if x + 2 is a factor of polynomials or identifying roots. By employing these methods, students gain a stronger grasp of polynomial equations and their factorizable forms, paving the way for more advanced algebraic manipulation.

Example: Find the value of \( k \), if \( x - 2 \) is a factor of \( 5x^3 + 4x^2 - 3x - 2 + k \).
Solution: As \( x - 2 \) is a factor of \( p(x) = 5x^3 + 4x^2 - 3x - 2 + k \), \( p(2) = 5(2)^3 + 4(2)^2 - 3(2) - 2 + k = 0 \)
Now,
\[ 5(2)^3 + 4(2)^2 - 3(2) - 2 + k = 0 \]
So,
\[ 5 \cdot 8 + 4 \cdot 4 - 6 - 2 + k = 0 \]
i.e.,
\[ k = -30 \]

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Audio Book

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Introduction to the Factor Theorem

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Let us now look at the situation of Example 10 above more closely. It tells us that since the remainder, 1/2q(−2) = 0, (2t + 1) is a factor of q(t), i.e., q(t) = (2t + 1)g(t) for some polynomial g(t). This is a particular case of the following theorem.

Factor Theorem: If p(x) is a polynomial of degree n > 1 and a is any real number, then (i) x − a is a factor of p(x), if p(a) = 0, and (ii) p(a) = 0, if x − a is a factor of p(x).

Detailed Explanation

The Factor Theorem establishes a direct relationship between the roots of a polynomial and its factors. In simple terms, if you can plug a number 'a' into the polynomial p(x) and get zero, then (x - a) is a factor of that polynomial. Alternatively, if (x - a) is a factor, then plugging 'a' into p(x) will yield zero. This is an essential tool for working with polynomials and determining their factors efficiently.

Examples & Analogies

Imagine you're looking for pieces that fit together to form a puzzle. If you find a piece that fits perfectly in a spot, you can say that this piece is part of the puzzle (it's a factor). In the context of polynomials, when you find a root by testing numbers, just like finding a fitting puzzle piece, it indicates that the polynomial can be divided by (x - that number).

Testing for Factors

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Example 6: Examine whether x + 2 is a factor of x3 + 3x2 + 5x + 6 and of 2x + 4.

Solution: The zero of x + 2 is −2. Let p(x) = x3 + 3x2 + 5x + 6 and s(x) = 2x + 4. Then, p(−2) = (−2)3 + 3(−2)2 + 5(−2) + 6 = −8 + 12 − 10 + 6 = 0. So, by the Factor Theorem, x + 2 is a factor of x3 + 3x2 + 5x + 6. Again, s(−2) = 2(−2) + 4 = 0. So, x + 2 is a factor of 2x + 4. In fact, you can check this without applying the Factor Theorem, since 2x + 4 = 2(x + 2).

Detailed Explanation

This example demonstrates how to apply the Factor Theorem to verify whether a certain polynomial is a factor of another polynomial. By substituting in the value of -2 into the polynomial (found from the factor x + 2), we can determine if it results in zero. If it does, according to the theorem, it confirms that the factor is valid. This process is an important skill when factorising polynomials.

Examples & Analogies

Think of it like a test for a secret club: the password is -2. If when you enter your password (substituting -2 into the polynomial), the door (the polynomial) swings open (results in zero), then you've got the correct password (the factor), and you’re part of the club (the polynomial can be divided by that factor).

Finding the Value of k

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Example 7: Find the value of k, if x − 1 is a factor of 4x3 + 3x2 − 4x + k.

Solution: As x − 1 is a factor of p(x) = 4x3 + 3x2 − 4x + k, p(1) = 0. Now, p(1) = 4(1)3 + 3(1)2 − 4(1) + k, so 4 + 3 − 4 + k = 0, i.e., k = −3.

Detailed Explanation

In this example, we need to find the constant 'k' such that when x - 1 is plugged into the polynomial, the result is zero. This means that k must be chosen so that the sum of the polynomial at x = 1 equals zero. By substituting 1 into the polynomial equation and solving for k, we find the value that allows x - 1 to be a factor.

Examples & Analogies

Imagine you're baking a cake and you want it to taste a certain way (here, being 'zero'). You know the ingredients (the polynomial terms) and one key ingredient (k). You adjust the amount of this ingredient to get the desired taste. Just like figuring out how to add k completes the recipe so it works perfectly, finding k ensures that x - 1 is indeed a valid factor.

Factoring Quadratic Polynomials

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We shall now try to factorise quadratic polynomials of the type ax2 + bx + c, where a ≠ 0 and a, b, c are constants. Factorisation of the polynomial ax2 + bx + c by splitting the middle term is as follows: Let its factors be (px + q) and (rx + s). Then ax2 + bx + c = (px + q)(rx + s) = prx2 + (ps + qr)x + qs.

Detailed Explanation

To factor a quadratic polynomial of the form ax² + bx + c, we need to find two values (let's call them p and q) such that they add up to b (the coefficient of x) and multiply to ac (the product of a and c). We can then express the polynomial as the product of two binomials, which reveals its factors.

Examples & Analogies

Think of it like breaking down a recipe that has certain portions. If you know how much sugar (p) and flour (q) you need (the middle term), and what total amount of ingredients you're using (the polynomial), you can figure out the proportions needed to make your dish (the factored form). Just like how combining properly measured ingredients gives you the right dish, combining the proper factors gives the correct polynomial.

Example of Factorisation

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Example 8: Factorise 6x2 + 17x + 5 by splitting the middle term and using the Factor Theorem.

Solution 1: (By splitting method): If we can find two numbers p and q such that p + q = 17 and pq = 6 × 5 = 30, then we can get the factors. So, let us look for the pairs of factors of 30. Some are 1 and 30, 2 and 15, 3 and 10, 5 and 6. Of these pairs, 2 and 15 will give us p + q = 17. Therefore, 6x2 + 17x + 5 = 6x2 + (2 + 15)x + 5 = 2x(3x + 1) + 5(3x + 1) = (3x + 1)(2x + 5).

Detailed Explanation

In the above example, we split the middle term based on two numbers we found that satisfied the required relationships. We then factored the terms and expressed the quadratic as a product of two binomials. This method efficiently reveals the factors of the polynomial, making it easier to analyze or solve.

Examples & Analogies

Imagine organizing a team for a game. If you have 6 players and you need to create teams of 2 and 5 players, you’ll combine them to form two teams. Just as this combination shows how many players will fit into those teams, factoring reveals the parts that make up the polynomial.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Factor Theorem: A method to determine factors of a polynomial based on its roots.

  • Polynomial Degree: The highest exponent of the polynomial determines its degree.

  • Quadratic Factorisation: Factor a quadratic by rewriting its middle term.

  • Cubic Factorisation: Requires finding one root to reduce the polynomial.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Example of factoring x² - 3x + 2 to show x-1 and x-2 as factors.

  • Example of applying the Factor Theorem to check if x + 2 is a factor of x³ + 3x² + 5x + 6.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Roots may be sweet, factors are neat; find one when you split, it’s a factoring hit!

📖 Fascinating Stories

  • Once there was a polynomial that wanted to find its factors. It remembered the Factor Theorem and finally discovered that every root could point towards a potential factor, and it joyously danced with its newfound friends, the factors.

🧠 Other Memory Gems

  • R.F. = Roots lead to Factors.

🎯 Super Acronyms

F.A.C.T. - Factorization And Checking Techniques.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Polynomial

    Definition:

    An algebraic expression composed of variables and coefficients, combined using addition, subtraction, multiplication, and non-negative integer exponents.

  • Term: Factor Theorem

    Definition:

    A theorem that states if a polynomial p(x) has a root at a, then (x - a) is a factor of p(x).

  • Term: Root

    Definition:

    A value of x that makes the polynomial p(x) equal to zero.

  • Term: Quadratic Polynomial

    Definition:

    A polynomial of degree two, expressed in the form ax² + bx + c.

  • Term: Cubic Polynomial

    Definition:

    A polynomial of degree three, expressed in the form ax³ + bx² + cx + d.

  • Term: Zero Polynomial

    Definition:

    A polynomial whose coefficients are all zero, often denoted as 0 and has no defined degree.