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Introduction
Factoring Quadratic Expressions helps you connect a mathematical rule to a result you can explain and check. Use the sum-product relationship for monic trinomials and split the middle term for a nonunit leading coefficient. Verify by expansion. This lesson belongs to exponents and polynomials and builds toward independent Algebra I problem solving.
Alignment uses 19 TAC §111.39(c). Review lessons sample their listed standards. Readiness and supporting labels describe STAAR emphasis, not whether a topic is required.
Learning objectives
- Identify the quantities and representations used in factoring quadratic expressions.
- Apply the lesson's rule to solve a numerical or contextual problem.
- Explain each mathematical step using the correct vocabulary.
- Verify an answer and correct a likely error.
Prerequisite knowledge
Use distribution, exponent rules, and integer factors.
Previous lesson: Factoring Common Factors
Key mathematical vocabulary
- coefficient
- A factor multiplying a variable term.
- polynomial
- A sum of terms with nonnegative integer variable exponents.
- factor
- An expression multiplied by another to form a product.
- quadratic
- A degree-two polynomial relationship.
- root
- A solution of an equation.
Formulas and rules
- a(b+c)=ab+ac
- Equivalent expressions agree for all allowed inputs.
- A solution satisfies every original condition.
Multiply, factor, and divide polynomials
Use the sum-product relationship for monic trinomials and split the middle term for a nonunit leading coefficient. Verify by expansion.
Distribution multiplies every term of one factor by every term of the other. Factoring reverses this process. For x²+bx+c, look for two numbers whose sum is b and product is c, then verify by expansion.
For ax²+bx+c with a≠1, find a factor pair of ac whose sum is b, split the middle term, and factor by grouping. If no rational pair works, real linear factors may still exist: use the discriminant to distinguish real-factorable quadratics from those with no real roots. Polynomial long division repeatedly divides leading terms, subtracts the resulting product, and brings down the remainder.
A method you can explain
- Read the question and identify the known information, unknown quantities, and any domain restrictions. Select a suitable representation and tool.
- Apply the relationship explained above. The needed numbers have sum 31 and product 228: 12 and 19.
- Write each calculation or logical inference on a separate line. Keep units and signs visible. When two methods are possible, compare which one uses the given information most directly.
- Check the result in the ORIGINAL conditions, not merely the last simplified line. Multiplying the factors (or divisor and quotient) recovers the polynomial.
- Interpret what the answer means. Decide whether its size, sign, and precision make sense; identify a restriction or assumption that would change the conclusion.
Five worked examples
Example 1 · Beginner
Factor x² + 31x + 228.
- The needed numbers have sum 31 and product 228: 12 and 19.
- Distribution gives x²+19x+12x+228=x²+31x+228.
- Thus x² + 31x + 228=(x + 12)(x + 19).
Answer: (x + 12)(x + 19)
Verify: Multiplying the factors (or divisor and quotient) recovers the polynomial.
Avoid this mistake: Checking only the product of a proposed factor pair.
Example 2 · Basic
Factor x² + 32x + 247.
- The needed numbers have sum 32 and product 247: 13 and 19.
- Distribution gives x²+19x+13x+247=x²+32x+247.
- Thus x² + 32x + 247=(x + 13)(x + 19).
Answer: (x + 13)(x + 19)
Verify: Multiplying the factors (or divisor and quotient) recovers the polynomial.
Avoid this mistake: Canceling terms across addition during division.
Example 3 · Intermediate
Factor 14x² + 267x + 19.
- Multiply the leading coefficient and constant: ac=266. Find split terms 266 and 1; their sum is 267.
- Split and group: 14x²+266x+x+19=14x(x+19)+1(x+19).
- Factor the shared binomial: (14x+1)(x+19).
Answer: (14x + 1)(x + 19)
Verify: Expanding produces the leading, middle, and constant terms.
Avoid this mistake: Forgetting to multiply the factors back to verify all coefficients.
Example 4 · Advanced
Factor 2x² + 41x + 20.
- Multiply the leading coefficient and constant: ac=40. Find split terms 40 and 1; their sum is 41.
- Split and group: 2x²+40x+x+20=2x(x+20)+1(x+20).
- Factor the shared binomial: (2x+1)(x+20).
Answer: (2x + 1)(x + 20)
Verify: Expanding produces the leading, middle, and constant terms.
Avoid this mistake: Checking only the product of a proposed factor pair.
Example 5 · Challenge
Factor 3x² + 61x + 20.
- Multiply the leading coefficient and constant: ac=60. Find split terms 60 and 1; their sum is 61.
- Split and group: 3x²+60x+x+20=3x(x+20)+1(x+20).
- Factor the shared binomial: (3x+1)(x+20).
Answer: (3x + 1)(x + 20)
Verify: Expanding produces the leading, middle, and constant terms.
Avoid this mistake: Canceling terms across addition during division.
Common mistakes and corrections
- Checking only the product of a proposed factor pair.
- Canceling terms across addition during division.
- Forgetting to multiply the factors back to verify all coefficients.
Real-world applications
- Find side lengths from a rectangular area expression. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
- Recover a missing dimension by dividing area by a known side. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
Topic summary
Use the sum-product relationship for monic trinomials and split the middle term for a nonunit leading coefficient. Verify by expansion.
Distribution multiplies every term of one factor by every term of the other. Factoring reverses this process. For x²+bx+c, look for two numbers whose sum is b and product is c, then verify by expansion.
Frequently asked questions
What should I know before factoring quadratic expressions?
Use distribution, exponent rules, and integer factors.
How can I check my work?
Multiplying the factors (or divisor and quotient) recovers the polynomial. A numerical check can detect an error; when claiming an identity for all inputs, also explain the algebraic property that makes it true.
Which mistakes should I watch for?
Checking only the product of a proposed factor pair. Canceling terms across addition during division. Forgetting to multiply the factors back to verify all coefficients.
Can I use another valid method?
Yes. Explain your assumptions, show enough intermediate work to justify the result, and verify it in the original problem. Equivalent expressions and valid alternative methods receive credit.
Mathematical process practice · A.1A–A.1G
- A.1A: Find side lengths from a rectangular area expression.
- A.1B: Plan, solve, check, and interpret the written response.
- A.1C: Select paper, a calculator, a table, or a graph; explain the choice.
- A.1D: Connect symbols with a verbal explanation and another representation.
- A.1E: Organize the solution with a table, diagram, or labeled equations.
- A.1F: Explain how the representation and the algebraic rule express the same relationship.
- A.1G: Justify each operation and the reasonableness of the conclusion.
Quick revision notes
- Use the sum-product relationship for monic trinomials and split the middle term for a nonunit leading coefficient. Verify by expansion.
- Checking only the product of a proposed factor pair.
- Canceling terms across addition during division.
- Forgetting to multiply the factors back to verify all coefficients.
Practice and explain
Try all four multiple-choice questions, then show your reasoning in the written response. Objective scores and AI feedback assess different skills.
Continue learning
Original Cambridge Veritas practice. These are not official STAAR items. Cambridge Veritas is not affiliated with or endorsed by TEA.
Official Algebra I TEKS (§111.39) · TEA assessed curriculum · STAAR blueprint · TEA instructional materials