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Introduction
Discriminant helps you connect a mathematical rule to a result you can explain and check. Use b²−4ac to classify real roots before calculating them. A zero discriminant gives one repeated real root. This lesson belongs to quadratic expressions and equations 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 discriminant.
- 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 signed arithmetic, factoring, square roots, and function notation.
Previous lesson: Quadratic Formula
Key mathematical vocabulary
- equation
- A statement that two expressions are equal.
- function
- A rule assigning one output to each permitted input.
- domain
- The set of permitted inputs.
- range
- The set of outputs produced.
- factor
- An expression multiplied by another to form a product.
Formulas and rules
- f(x)=ax²+bx+c, a≠0
- f(x)=a(x−h)²+k
- x=(−b±√(b²−4ac))/(2a)
Quadratic functions and equations
Use b²−4ac to classify real roots before calculating them. A zero discriminant gives one repeated real root.
A quadratic function has the form f(x)=ax²+bx+c with a≠0. Its graph is a parabola. Vertex form f(x)=a(x−h)²+k identifies the vertex (h,k), axis x=h, and opening direction from the sign of a.
Factored form connects roots with linear factors: a(x−r₁)(x−r₂) has zeros r₁ and r₂. Solve real quadratic equations by factoring, square roots, completing the square, or the quadratic formula. The discriminant b²−4ac determines whether there are two, one repeated, or no real roots. A quadratic’s mathematical domain is all real numbers, while its range is bounded by its extreme value.
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. Identify A=1, B=24, C=168.
- 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. Completing the square gives (x+a)²=−b, also impossible over the reals.
- 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
Find the discriminant of x² + 24x + 168=0 and state its number of real roots.
- Identify A=1, B=24, C=168.
- Compute D=B²−4AC=576−672=-96.
- Since D<0, no real square root of D exists, so the equation has no real roots. The parabola does not cross the x-axis.
Answer: -96; no real roots
Verify: Completing the square gives (x+a)²=−b, also impossible over the reals.
Avoid this mistake: Dropping the ± when solving a squared equation.
Example 2 · Basic
Find the discriminant of x² + 26x + 193=0 and state its number of real roots.
- Identify A=1, B=26, C=193.
- Compute D=B²−4AC=676−772=-96.
- Since D<0, no real square root of D exists, so the equation has no real roots. The parabola does not cross the x-axis.
Answer: -96; no real roots
Verify: Completing the square gives (x+a)²=−b, also impossible over the reals.
Avoid this mistake: Reading the vertex sign incorrectly from (x−h)².
Example 3 · Intermediate
Find the discriminant of x² + 28x + 220=0 and state its number of real roots.
- Identify A=1, B=28, C=220.
- Compute D=B²−4AC=784−880=-96.
- Since D<0, no real square root of D exists, so the equation has no real roots. The parabola does not cross the x-axis.
Answer: -96; no real roots
Verify: Completing the square gives (x+a)²=−b, also impossible over the reals.
Avoid this mistake: Assuming every parabola crosses the x-axis twice.
Example 4 · Advanced
Find the discriminant of x² + 4x + 29=0 and state its number of real roots.
- Identify A=1, B=4, C=29.
- Compute D=B²−4AC=16−116=-100.
- Since D<0, no real square root of D exists, so the equation has no real roots. The parabola does not cross the x-axis.
Answer: -100; no real roots
Verify: Completing the square gives (x+a)²=−b, also impossible over the reals.
Avoid this mistake: Dropping the ± when solving a squared equation.
Example 5 · Challenge
Find the discriminant of x² + 6x + 34=0 and state its number of real roots.
- Identify A=1, B=6, C=34.
- Compute D=B²−4AC=36−136=-100.
- Since D<0, no real square root of D exists, so the equation has no real roots. The parabola does not cross the x-axis.
Answer: -100; no real roots
Verify: Completing the square gives (x+a)²=−b, also impossible over the reals.
Avoid this mistake: Reading the vertex sign incorrectly from (x−h)².
Common mistakes and corrections
- Dropping the ± when solving a squared equation.
- Reading the vertex sign incorrectly from (x−h)².
- Assuming every parabola crosses the x-axis twice.
Real-world applications
- Model the height of a thrown object over its physical time interval. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
- Find maximum area or minimum cost from a quadratic model. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
Topic summary
Use b²−4ac to classify real roots before calculating them. A zero discriminant gives one repeated real root.
A quadratic function has the form f(x)=ax²+bx+c with a≠0. Its graph is a parabola. Vertex form f(x)=a(x−h)²+k identifies the vertex (h,k), axis x=h, and opening direction from the sign of a.
Frequently asked questions
What should I know before discriminant?
Use signed arithmetic, factoring, square roots, and function notation.
How can I check my work?
Completing the square gives (x+a)²=−b, also impossible over the reals. 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?
Dropping the ± when solving a squared equation. Reading the vertex sign incorrectly from (x−h)². Assuming every parabola crosses the x-axis twice.
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: Model the height of a thrown object over its physical time interval.
- 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 b²−4ac to classify real roots before calculating them. A zero discriminant gives one repeated real root.
- Dropping the ± when solving a squared equation.
- Reading the vertex sign incorrectly from (x−h)².
- Assuming every parabola crosses the x-axis twice.
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.
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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