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Introduction
Writing Algebraic Expressions helps you connect a mathematical rule to a result you can explain and check. Translate fixed charges and repeated charges separately. Define the variable and state a meaningful input restriction. This lesson belongs to algebra foundations 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 writing algebraic 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
Identify variables, constants, and multiplication.
Previous lesson: Algebraic Expressions
Key mathematical vocabulary
- variable
- A letter representing an unknown or changing quantity.
- ratio
- A comparison by division.
- rate
- A ratio measured in units per unit.
- input
- A value supplied to a rule.
- expression
- A mathematical phrase representing a value.
Formulas and rules
- a(b+c)=ab+ac
- Equivalent expressions agree for all allowed inputs.
- A solution satisfies every original condition.
Translate words into algebra
Translate fixed charges and repeated charges separately. Define the variable and state a meaningful input restriction.
Choose a letter and define its units before writing a rule. “Per,” “each,” and “times” indicate multiplication. “More than” usually adds a quantity; “less than” reverses the order in subtraction. Total cost often combines a fixed fee with a variable charge.
Translate one relationship at a time. If each item costs r dollars and there are n items, rn has units dollars. Adding a fee b gives rn+b. Parentheses mean that an operation applies to an entire group: r(n+b) is a different situation.
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. Let v count visits; v is a nonnegative whole number.
- 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. For 2 visits, the cost is 12(2)+27=$51. At zero visits it is $27.
- 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
A club charges $27 to join and $12 per visit. Write the total cost for v visits.
- Let v count visits; v is a nonnegative whole number.
- The repeated charge is 12 times v, or 12v.
- Add the one-time charge 27; the total is 12v + 27 dollars.
Answer: 12v + 27
Verify: For 2 visits, the cost is 12(2)+27=$51. At zero visits it is $27.
Avoid this mistake: Reversing subtraction: five less than x is x−5.
Example 2 · Basic
A club charges $27 to join and $13 per visit. Write the total cost for v visits.
- Let v count visits; v is a nonnegative whole number.
- The repeated charge is 13 times v, or 13v.
- Add the one-time charge 27; the total is 13v + 27 dollars.
Answer: 13v + 27
Verify: For 2 visits, the cost is 13(2)+27=$53. At zero visits it is $27.
Avoid this mistake: Multiplying the fixed fee by quantity: add it once.
Example 3 · Intermediate
A club charges $27 to join and $14 per visit. Write the total cost for v visits.
- Let v count visits; v is a nonnegative whole number.
- The repeated charge is 14 times v, or 14v.
- Add the one-time charge 27; the total is 14v + 27 dollars.
Answer: 14v + 27
Verify: For 2 visits, the cost is 14(2)+27=$55. At zero visits it is $27.
Avoid this mistake: Ignoring units: add only quantities measured in the same units.
Example 4 · Advanced
A club charges $28 to join and $2 per visit. Write the total cost for v visits.
- Let v count visits; v is a nonnegative whole number.
- The repeated charge is 2 times v, or 2v.
- Add the one-time charge 28; the total is 2v + 28 dollars.
Answer: 2v + 28
Verify: For 2 visits, the cost is 2(2)+28=$32. At zero visits it is $28.
Avoid this mistake: Reversing subtraction: five less than x is x−5.
Example 5 · Challenge
A club charges $28 to join and $3 per visit. Write the total cost for v visits.
- Let v count visits; v is a nonnegative whole number.
- The repeated charge is 3 times v, or 3v.
- Add the one-time charge 28; the total is 3v + 28 dollars.
Answer: 3v + 28
Verify: For 2 visits, the cost is 3(2)+28=$34. At zero visits it is $28.
Avoid this mistake: Multiplying the fixed fee by quantity: add it once.
Common mistakes and corrections
- Reversing subtraction: five less than x is x−5.
- Multiplying the fixed fee by quantity: add it once.
- Ignoring units: add only quantities measured in the same units.
Real-world applications
- Model an admission fee plus game tokens. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
- Translate a recipe that needs a fixed topping and a repeated amount per serving. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
Topic summary
Translate fixed charges and repeated charges separately. Define the variable and state a meaningful input restriction.
Choose a letter and define its units before writing a rule. “Per,” “each,” and “times” indicate multiplication. “More than” usually adds a quantity; “less than” reverses the order in subtraction. Total cost often combines a fixed fee with a variable charge.
Frequently asked questions
What should I know before writing algebraic expressions?
Identify variables, constants, and multiplication.
How can I check my work?
For 2 visits, the cost is 12(2)+27=$51. At zero visits it is $27. 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?
Reversing subtraction: five less than x is x−5. Multiplying the fixed fee by quantity: add it once. Ignoring units: add only quantities measured in the same units.
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 an admission fee plus game tokens.
- 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
- Translate fixed charges and repeated charges separately. Define the variable and state a meaningful input restriction.
- Reversing subtraction: five less than x is x−5.
- Multiplying the fixed fee by quantity: add it once.
- Ignoring units: add only quantities measured in the same units.
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