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Introduction
Decay Factor helps you connect a mathematical rule to a result you can explain and check. A decay factor equals 1 minus the decimal decrease rate. Interpret the remaining fraction after each period. This lesson belongs to exponential functions 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 decay factor.
- 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
Evaluate powers and convert percentages to decimals.
Previous lesson: Growth Factor
Key mathematical vocabulary
- constant
- A value fixed within a problem.
- domain
- The set of permitted inputs.
- range
- The set of outputs produced.
- exponent
- A number specifying a power.
- factor
- An expression multiplied by another to form a product.
Formulas and rules
- f(x)=abx, b>0, b≠1
- Growth factor=1+r; decay factor=1−r
- A=P(1+r/n)nt
Exponential growth and decay
A decay factor equals 1 minus the decimal decrease rate. Interpret the remaining fraction after each period.
An exponential model has the form f(t)=abt, where b>0 and b≠1 for a nonconstant exponential. The initial value is a=f(0). Each increase of 1 in t multiplies the output by b rather than adding a constant.
For a positive initial amount, b>1 models growth and 0<b<1 models decay. A rate r written as a decimal gives growth factor 1+r or decay factor 1−r. Compound interest is A=P(1+r/n)nt, with compatible units. For a≠0, the mathematical domain is all real numbers; the sign of a determines whether the range is y>0 or y<0. Context may restrict time to t≥0.
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. Read the base as the fraction remaining: 0.82.
- 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. The remaining fraction plus the lost fraction equals the whole.
- 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 quantity follows Q(t)=120(0.82)t. Give the fraction remaining per period and the percent decrease.
- Read the base as the fraction remaining: 0.82.
- The fraction lost is 1−0.82=0.18.
- Convert to a percent: 0.18·100=18%. A factor below 1 indicates decay.
Answer: 0.82; 18% decrease
Verify: The remaining fraction plus the lost fraction equals the whole.
Avoid this mistake: Using the percent rate itself as the growth factor.
Example 2 · Basic
A quantity follows Q(t)=130(0.82)t. Give the fraction remaining per period and the percent decrease.
- Read the base as the fraction remaining: 0.82.
- The fraction lost is 1−0.82=0.18.
- Convert to a percent: 0.18·100=18%. A factor below 1 indicates decay.
Answer: 0.82; 18% decrease
Verify: The remaining fraction plus the lost fraction equals the whole.
Avoid this mistake: Adding the same amount each period in an exponential model.
Example 3 · Intermediate
A quantity follows Q(t)=140(0.82)t. Give the fraction remaining per period and the percent decrease.
- Read the base as the fraction remaining: 0.82.
- The fraction lost is 1−0.82=0.18.
- Convert to a percent: 0.18·100=18%. A factor below 1 indicates decay.
Answer: 0.82; 18% decrease
Verify: The remaining fraction plus the lost fraction equals the whole.
Avoid this mistake: Including zero in the range of an unshifted nonzero exponential.
Example 4 · Advanced
A quantity follows Q(t)=20(0.81)t. Give the fraction remaining per period and the percent decrease.
- Read the base as the fraction remaining: 0.81.
- The fraction lost is 1−0.81=0.19.
- Convert to a percent: 0.19·100=19%. A factor below 1 indicates decay.
Answer: 0.81; 19% decrease
Verify: The remaining fraction plus the lost fraction equals the whole.
Avoid this mistake: Using the percent rate itself as the growth factor.
Example 5 · Challenge
A quantity follows Q(t)=30(0.81)t. Give the fraction remaining per period and the percent decrease.
- Read the base as the fraction remaining: 0.81.
- The fraction lost is 1−0.81=0.19.
- Convert to a percent: 0.19·100=19%. A factor below 1 indicates decay.
Answer: 0.81; 19% decrease
Verify: The remaining fraction plus the lost fraction equals the whole.
Avoid this mistake: Adding the same amount each period in an exponential model.
Common mistakes and corrections
- Using the percent rate itself as the growth factor.
- Adding the same amount each period in an exponential model.
- Including zero in the range of an unshifted nonzero exponential.
Real-world applications
- Model a population with a constant percent change. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
- Predict a repeated depreciation or compound-interest balance. Define the input and its units, apply the mathematical relationship, and check that the result fits the setting.
Topic summary
A decay factor equals 1 minus the decimal decrease rate. Interpret the remaining fraction after each period.
An exponential model has the form f(t)=abt, where b>0 and b≠1 for a nonconstant exponential. The initial value is a=f(0). Each increase of 1 in t multiplies the output by b rather than adding a constant.
Frequently asked questions
What should I know before decay factor?
Evaluate powers and convert percentages to decimals.
How can I check my work?
The remaining fraction plus the lost fraction equals the whole. 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?
Using the percent rate itself as the growth factor. Adding the same amount each period in an exponential model. Including zero in the range of an unshifted nonzero exponential.
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 a population with a constant percent change.
- 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
- A decay factor equals 1 minus the decimal decrease rate. Interpret the remaining fraction after each period.
- Using the percent rate itself as the growth factor.
- Adding the same amount each period in an exponential model.
- Including zero in the range of an unshifted nonzero exponential.
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