Variation laws, constants and original domains
Fit a stipulated variation law, keep its real input restrictions and explain how changing an input changes its output.
Exam scope and assessment notes
Original selected variation teaching with four stages,eight tasks,26named prediction traces and18recovery targets. Calibrate the eight listed families from retained original observations and actual equation operations. Keep natural and stated input restrictions,all real recoveries,original-law return checks,change factors and coefficient dimensions. Exact irrational values remain defined and must be entered symbolically;selected equivalent square/cube-root values are compared exactly. More general symbolic coefficients/forms need human review. A complete incorrect attempt may study the explained reference without passing fixed comparisons. Written formulas,derivations,whole-domain arguments,unit prose and empirical model validity remain unreviewed. Finite observations do not prove a physical law. All14Functions source rows have reviewed structural teaching/practice links;other Mathematics B source gaps remain explicit in the current course map;wider Functions and remaining Pearson teaching/practice are incomplete. No official marks,mastery,specialist approval or formal assessment readiness. Single Higher-tier linear4MB1,entry context2026/27 only. ISBN9780435044107 is metadata only;no verified book body,chapter or Hub mapping and no MathematicsA,Foundation or modular inheritance.
Before you begin
- Exact arithmetic,integer and fractional powers,principal roots,solving equations,real-domain restrictions and unit conversions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For a nonzero-coefficient inverse law y=k/x,which expression keeps the same value at every allowed input?
What you will learn
- Translate the eight listed direct and inverse variation forms into a model with a constant.
- Use an admitted observation to identify a coefficient,or explain why the observation leaves it undetermined.
- Predict exact outputs and recover every allowed real input,retaining square-root signs and denominator restrictions.
- Explain change ratios and coefficient units without treating finite data as proof of an empirical law.
Each stage starts with blank work and independent answer fields. Changing stage begins a fresh attempt.
Variation laws and original domains: Worked
Choose the stipulated variation family. Show your original calibration substitution and actual equation operations,then predict outputs,recover every permitted input and explain the change ratio. Enter your final answer separately.
For sets,use (-inf,inf),intervals such as [0,2),unions with U and none for an empty set. Type none for no available value or no unit. In a finite recovery list,add one row for each candidate;zero rows means no candidates. “Every original allowed input” is different from a finite empty list.
Start your calibration with an original substituted equation. Add an equation row for each real operation you perform. The unknown coefficient is k;use * for multiplication. Exact values may use sqrt(...) and selected cbrt(...) notation. Keep a real irrational prediction exact instead of rounding it or declaring it undefined.
The coefficient-unit fields describe the output unit multiplied by the input unit to your entered exponent. A ratio requires both original inputs to be admitted and a nonzero original output. Written formulas,derivations,whole-domain arguments,unit prose and empirical model validity always need human review. Fixed comparisons do not approve those sentences or award marks or mastery.
Two abstract variables follow stipulated laws on their real natural domains. First y is directly proportional to x, and y=12 when x=8. Then y is inversely proportional to x, and y=15 when x=4. For each law, identify the coefficient, retain the real input domain, predict all named inputs, recover every input for the requested output, and explain the change ratio. No numerical constant is supplied.
Task 1: calibrate and use the original law
Stipulated law: Direct: y = kx. Find the numerical coefficient from the observation.
Original observations: input 8 gives output 12.
Stated input restriction: (-inf,inf). Retain any additional denominator and principal-root conditions in the original law.
Prediction inputs: 20, 0, -4. Recovery outputs: 9, 0. Change original input 8 by multiplication factor 3/2.
Original input unit: dimensionless. Original output unit: dimensionless.
Inspect my entered work
Only your own current entries appear here. This display supplies no coefficient,calculation,domain,recovery or change ratio.
My chosen law,coefficient and original real domain
- My variation family
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- My coefficient k
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- My allowed original real input set
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My coefficient dimensions
- Original input unit (or none)
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- Original output unit (or none)
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- Exponent of the input unit in the coefficient unit
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My calibration using input 8 and output 12
- Original observation input
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- Original observation output
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- My value of the proportional basis
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- Explain my substitution and nonzero divisor
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My prediction at original input 20
- My original input classification
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- Is the original law defined at this input?
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- My exact predicted output (or none)
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- Explain my original-domain and value decision
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My prediction at original input 0
- My original input classification
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- Is the original law defined at this input?
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- My exact predicted output (or none)
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- Explain my original-domain and value decision
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My prediction at original input -4
- My original input classification
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- Is the original law defined at this input?
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- My exact predicted output (or none)
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- Explain my original-domain and value decision
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My recovery for output 9
- My complete recovery classification
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- Explain all real recoveries and original restrictions
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My recovery for output 0
- My complete recovery classification
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- Explain all real recoveries and original restrictions
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My change from original input 8 by factor 3/2
- Original input for the change
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- Original input multiplication factor
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- My changed original input
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- My output before the change (or none)
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- My output after the change (or none)
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- My change-ratio classification
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- My exact output change ratio (or none)
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- Explain the input restrictions and baseline decision
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My original model derivation and limitations
- Derive my stipulated model and coefficient without changing its domain
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- Explain the domain,recovery,units and limits of this stipulated model
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Work status: Not recorded
My separate final answer
Enter your model,coefficient,domain,coefficient dimensions,predictions,complete recoveries and change ratio independently. Your work entries are not copied into these fields.
Task 2: calibrate and use the original law
Stipulated law: Inverse: y = k/x. Find the numerical coefficient from the observation.
Original observations: input 4 gives output 15.
Stated input restriction: (-inf,inf). Retain any additional denominator and principal-root conditions in the original law.
Prediction inputs: 10, 0, -4. Recovery outputs: 12, 0. Change original input 4 by multiplication factor 2.
Original input unit: dimensionless. Original output unit: dimensionless.
Inspect my entered work
Only your own current entries appear here. This display supplies no coefficient,calculation,domain,recovery or change ratio.
My chosen law,coefficient and original real domain
- My variation family
- Not entered
- My coefficient k
- Not entered
- My allowed original real input set
- Not entered
My coefficient dimensions
- Original input unit (or none)
- Not entered
- Original output unit (or none)
- Not entered
- Exponent of the input unit in the coefficient unit
- Not entered
My calibration using input 4 and output 15
- Original observation input
- Not entered
- Original observation output
- Not entered
- My value of the proportional basis
- Not entered
- Explain my substitution and nonzero divisor
- Not entered
My prediction at original input 10
- My original input classification
- Not entered
- Is the original law defined at this input?
- Not entered
- My exact predicted output (or none)
- Not entered
- Explain my original-domain and value decision
- Not entered
My prediction at original input 0
- My original input classification
- Not entered
- Is the original law defined at this input?
- Not entered
- My exact predicted output (or none)
- Not entered
- Explain my original-domain and value decision
- Not entered
My prediction at original input -4
- My original input classification
- Not entered
- Is the original law defined at this input?
- Not entered
- My exact predicted output (or none)
- Not entered
- Explain my original-domain and value decision
- Not entered
My recovery for output 12
- My complete recovery classification
- Not entered
- Explain all real recoveries and original restrictions
- Not entered
My recovery for output 0
- My complete recovery classification
- Not entered
- Explain all real recoveries and original restrictions
- Not entered
My change from original input 4 by factor 2
- Original input for the change
- Not entered
- Original input multiplication factor
- Not entered
- My changed original input
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- My output before the change (or none)
- Not entered
- My output after the change (or none)
- Not entered
- My change-ratio classification
- Not entered
- My exact output change ratio (or none)
- Not entered
- Explain the input restrictions and baseline decision
- Not entered
My original model derivation and limitations
- Derive my stipulated model and coefficient without changing its domain
- Not entered
- Explain the domain,recovery,units and limits of this stipulated model
- Not entered
Work status: Not recorded
My separate final answer
Enter your model,coefficient,domain,coefficient dimensions,predictions,complete recoveries and change ratio independently. Your work entries are not copied into these fields.
Record every current original variation construction table.
Choose the stipulated law before finding its constant
A proportion statement specifies how y depends on x. Write y=kx for direct proportion and y=k/x for inverse proportion. More generally the listed laws use x,x²,x³,√x or the corresponding reciprocal as the proportional basis. A constant coefficient multiplies that whole basis; an added offset changes the model.
Calibration means inserting an observed pair into the stated model. Divide y by the nonzero basis value to identify k. The pair(0,0) in a direct power or root law cannot identify k:every value of k fits that pair. Inconsistent observations must not be averaged into an exact law.
Keep the real domain and all recoveries
The integer-power direct laws allow all real abstract inputs. Reciprocal laws exclude zero. A principal square root requires x≥0;its reciprocal requires x>0. A practical restriction can narrow these sets further. Even coefficient zero does not restore a denominator-zero or non-real original input.
For a square or reciprocal-square law,solving for x may give both signs on an unrestricted abstract domain. Retain every allowed candidate and substitute it into the original law. A principal-root equation also has a sign condition before squaring. A transformed equation alone cannot authorize a candidate rejected by the original expression.
Use powers to explain change factors
If an admitted nonzero input is multiplied by a factor a,its square and cube multiply by a² and a³. Reciprocals invert these factors. On admitted nonnegative square-root inputs,the root factor is√a;the reciprocal-root factor is1/√a. Check that both the original and changed inputs remain admitted before using a change ratio.
A numerical ratio y(new)/y(old) is unavailable when the original output is zero. This includes a zero coefficient and the admitted origin in a direct law. It does not automatically make the output value undefined. A negative scaling factor can be permitted for abstract integer-power laws while violating a root or practical domain.
State units and the limits of calibration
The coefficient has units that make k times the basis agree with the output unit. For example,r=k√t gives k the unit reading per√second,whereas r=k/√t gives reading times√second. Changing units changes the numerical value of the coefficient.
In these exercises a law is stipulated. Finite observations can contradict that law or fit it consistently,but matching them does not establish a physical law over all unobserved inputs. Explain the assumptions and operating range of any practical model. Exact irrational predictions remain real;use a symbolic expression rather than substituting a rounded value.
Sources for this lesson
Pause and explain
For y=2x² with original domain R,which complete recovery set gives output50?
Worked example
Two abstract variables follow stipulated laws on their real natural domains. First y is directly proportional to x, and y=12 when x=8. Then y is inversely proportional to x, and y=15 when x=4. For each law, identify the coefficient, retain the real input domain, predict all named inputs, recover every input for the requested output, and explain the change ratio. No numerical constant is supplied.
Show the worked solution
- For direct proportion, write y=kx and substitute the original observation:12=8k, so k=3/2. Thus y=(3/2)x for every real x in this abstract task. At x=20,0,−4 the outputs are30,0,−6. Solving9=(3/2)x gives x=6; substituting6 returns9.
- When an admitted nonzero input is multiplied by3/2, y=(3/2)x is also multiplied by3/2. At input zero the numerical ratio y(new)/y(old) is unavailable because its denominator is zero; this does not make the direct law undefined at zero.
- For inverse proportion, use y=k/x, so the invariant is xy=k. The pair(4,15) gives k=60 and y=60/x with x≠0. Inputs10 and−4 give6 and−15; x=0 remains excluded, even though the phrase “inverse” does not itself describe a recovery function.
- To recover an input for y=12, solve12x=60 and retain x=5, which returns12. Doubling an allowed nonzero x multiplies y by1/2. The product xy stays60; the quotient y/x does not stay constant.
- In both tasks the law was stipulated before calibration. A finite collection of matching observations checks consistency with that law; it does not prove that an unknown physical relationship is proportional for all inputs.
Answer For direct proportion, write y=kx and substitute the original observation:12=8k, so k=3/2. Thus y=(3/2)x for every real x in this abstract task. At x=20,0,−4 the outputs are30,0,−6. Solving9=(3/2)x gives x=6; substituting6 returns9. When an admitted nonzero input is multiplied by3/2, y=(3/2)x is also multiplied by3/2. At input zero the numerical ratio y(new)/y(old) is unavailable because its denominator is zero; this does not make the direct law undefined at zero. For inverse proportion, use y=k/x, so the invariant is xy=k. The pair(4,15) gives k=60 and y=60/x with x≠0. Inputs10 and−4 give6 and−15; x=0 remains excluded, even though the phrase “inverse” does not itself describe a recovery function. To recover an input for y=12, solve12x=60 and retain x=5, which returns12. Doubling an allowed nonzero x multiplies y by1/2. The product xy stays60; the quotient y/x does not stay constant. In both tasks the law was stipulated before calibration. A finite collection of matching observations checks consistency with that law; it does not prove that an unknown physical relationship is proportional for all inputs.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
One stipulated law has y proportional to x², with y=18 when x=3. Another has y proportional to 1/x², with y=12 when x=2. Their abstract inputs may have either sign where the original rule is real. Find each coefficient and formula, predict the named inputs, retain every real recovery for y=50 or y=3 respectively, and explain the effect of multiplying x by−2. Explain why the calibration pair (0,0) alone would not identify the first coefficient.
Give me a hint
Insert the observation into the whole square or reciprocal-square basis before dividing. Preserve both allowed recovery signs.
Compare my reasoning
- For the square law,18=k·3² gives k=2, so y=2x² on R. At x=−4,0,2 the outputs are32,0,8. The negative input is squared; it does not create a negative output here.
- Recovering y=50 gives x²=25, so x=−5 and x=5 are both allowed and both return50. Choosing only the positive input would silently change the original domain. In contrast, y=0 has just x=0. The pair(0,0) gives0=k·0 and leaves k undetermined.
- For the reciprocal-square law,12=k/2² gives k=48. Thus y=48/x² on R except0. At x=−4 and3 the outputs are3 and16/3. Input0 is excluded before any simplification.
- Recovering y=3 gives x²=16, so retain both−4 and4, each nonzero and each returning3. Output0 cannot occur with coefficient48 at any finite allowed input.
- Multiplying x by−2 multiplies x² by4. The square-law output therefore multiplies by4, and the reciprocal-square output by1/4. The sign change does not make either squared output negative. A numerical change ratio requires a nonzero original output.
For the square law,18=k·3² gives k=2, so y=2x² on R. At x=−4,0,2 the outputs are32,0,8. The negative input is squared; it does not create a negative output here. Recovering y=50 gives x²=25, so x=−5 and x=5 are both allowed and both return50. Choosing only the positive input would silently change the original domain. In contrast, y=0 has just x=0. The pair(0,0) gives0=k·0 and leaves k undetermined. For the reciprocal-square law,12=k/2² gives k=48. Thus y=48/x² on R except0. At x=−4 and3 the outputs are3 and16/3. Input0 is excluded before any simplification. Recovering y=3 gives x²=16, so retain both−4 and4, each nonzero and each returning3. Output0 cannot occur with coefficient48 at any finite allowed input. Multiplying x by−2 multiplies x² by4. The square-law output therefore multiplies by4, and the reciprocal-square output by1/4. The sign change does not make either squared output negative. A numerical change ratio requires a nonzero original output.
Look for these in your work
- Retain the stipulated original variation family and all real/stated input restrictions.
- Show the original calibration substitution and actual equation operations used to identify the coefficient.
- Predict every named input exactly,and give every allowed recovery candidate with an original-law return check.
- Explain the output change factor only when both inputs are admitted and the original output is nonzero.
- State coefficient dimensions and model assumptions;written formulas,derivations and empirical validity need human review.
A stipulated cube law has y=−54 at x=−3. A stipulated reciprocal-cube law has y=−3 at x=−2. Retain the signs when finding each coefficient, evaluating the named inputs and recovering x for y=−16 or y=3 respectively. Explain the output ratio when x is multiplied by−1/2. Decide whether output zero is attainable for each original law.
Give me a hint
A negative number cubed stays negative. Separate the sign of the coefficient from the sign of the input.
Compare my reasoning
- For the cube law,−54=k(−3)³=−27k, so k=2 and y=2x³. At x=4,0,−1/2 the outputs are128,0,−1/4. Cubing keeps the sign of an abstract input.
- Recovering y=−16 gives x³=−8, hence the unique real input x=−2. It returns−16. The two signs used for square roots must not be attached to a real cube root. Output0 is attained only at x=0.
- For the reciprocal-cube law,−3=k/(−2)³=−k/8, so k=24 and y=24/x³ with x≠0. At x=3 and−1/2 the outputs are8/9 and−192; x=0 remains outside the original domain.
- Recovering y=3 gives x³=8, so x=2. No finite nonzero input produces output0 with nonzero coefficient24.
- The input factor−1/2 cubes to−1/8. It multiplies the direct-cube output by−1/8 and the reciprocal-cube output by−8. These signed ratios refer to the stipulated abstract domains, not to an undeclared extension of a nonnegative physical quantity.
For the cube law,−54=k(−3)³=−27k, so k=2 and y=2x³. At x=4,0,−1/2 the outputs are128,0,−1/4. Cubing keeps the sign of an abstract input. Recovering y=−16 gives x³=−8, hence the unique real input x=−2. It returns−16. The two signs used for square roots must not be attached to a real cube root. Output0 is attained only at x=0. For the reciprocal-cube law,−3=k/(−2)³=−k/8, so k=24 and y=24/x³ with x≠0. At x=3 and−1/2 the outputs are8/9 and−192; x=0 remains outside the original domain. Recovering y=3 gives x³=8, so x=2. No finite nonzero input produces output0 with nonzero coefficient24. The input factor−1/2 cubes to−1/8. It multiplies the direct-cube output by−1/8 and the reciprocal-cube output by−8. These signed ratios refer to the stipulated abstract domains, not to an undeclared extension of a nonnegative physical quantity.
Look for these in your work
- Retain the stipulated original variation family and all real/stated input restrictions.
- Show the original calibration substitution and actual equation operations used to identify the coefficient.
- Predict every named input exactly,and give every allowed recovery candidate with an original-law return check.
- Explain the output change factor only when both inputs are admitted and the original output is nonzero.
- State coefficient dimensions and model assumptions;written formulas,derivations and empirical validity need human review.
Two separate hypothetical instruments use a time setting t in seconds and report r in reading units. The first model stipulates r proportional to √t for t≥0, with r=6 at t=9. The second stipulates r proportional to 1/√t for t>0, with r=9 at t=4. Derive each coefficient with its units, predict the named settings including t=2 exactly, recover every allowed time for r=10 or r=3 respectively, and compare the change when time is multiplied by9/4. Explain why a negative requested reading cannot be recovered by squaring it, and why zero time is admitted in only the first model. These are stated mathematical models, not claims about real instruments.
Give me a hint
Write the principal-root sign restriction before squaring,and express the prediction at time2 exactly.
Compare my reasoning
- For the first instrument model,6=k√9=3k gives k=2 reading units per√second. Its rule is r=2√t for t≥0. At t=16,2,0 the outputs are8,2√2,0. The value2√2 is real and exact; replacing it by a rounded decimal or declaring it undefined loses information. Setting t=−1 is outside the real domain.
- Recovering r=10 gives √t=5 and t=25 seconds; substitution returns10. A requested r=−2 is impossible because k is positive and the principal square root is nonnegative. Squaring the impossible equation would propose t=1, whose original reading is+2, not−2.
- For the second model,9=k/√4=k/2 gives k=18 reading units times√second. Its rule is r=18/√t for t>0. At t=9 and2 the exact outputs are6 and18/√2=9√2. Negative time is not real here, and zero time is forbidden by the original denominator.
- Recovering r=3 gives √t=6 and t=36 seconds; substitution returns3. A nonpositive reading has no finite allowed time in this positive-coefficient model. These conclusions follow from its stated restrictions and principal root, not from squaring alone.
- Multiplying an allowed positive time by9/4 multiplies its square root by3/2. The first output is multiplied by3/2 and the second by2/3. At first-model time0 the output ratio is unavailable, although the output itself remains0. The fitted coefficient carries different units in the two laws and changes numerically when the chosen units change.
- The instrument laws are hypothetical assumptions. Checking the one calibration and predictions does not establish either law experimentally or justify using it outside the model domain.
For the first instrument model,6=k√9=3k gives k=2 reading units per√second. Its rule is r=2√t for t≥0. At t=16,2,0 the outputs are8,2√2,0. The value2√2 is real and exact; replacing it by a rounded decimal or declaring it undefined loses information. Setting t=−1 is outside the real domain. Recovering r=10 gives √t=5 and t=25 seconds; substitution returns10. A requested r=−2 is impossible because k is positive and the principal square root is nonnegative. Squaring the impossible equation would propose t=1, whose original reading is+2, not−2. For the second model,9=k/√4=k/2 gives k=18 reading units times√second. Its rule is r=18/√t for t>0. At t=9 and2 the exact outputs are6 and18/√2=9√2. Negative time is not real here, and zero time is forbidden by the original denominator. Recovering r=3 gives √t=6 and t=36 seconds; substitution returns3. A nonpositive reading has no finite allowed time in this positive-coefficient model. These conclusions follow from its stated restrictions and principal root, not from squaring alone. Multiplying an allowed positive time by9/4 multiplies its square root by3/2. The first output is multiplied by3/2 and the second by2/3. At first-model time0 the output ratio is unavailable, although the output itself remains0. The fitted coefficient carries different units in the two laws and changes numerically when the chosen units change. The instrument laws are hypothetical assumptions. Checking the one calibration and predictions does not establish either law experimentally or justify using it outside the model domain.
Look for these in your work
- Retain the stipulated original variation family and all real/stated input restrictions.
- Show the original calibration substitution and actual equation operations used to identify the coefficient.
- Predict every named input exactly,and give every allowed recovery candidate with an original-law return check.
- Explain the output change factor only when both inputs are admitted and the original output is nonzero.
- State coefficient dimensions and model assumptions;written formulas,derivations and empirical validity need human review.
Common mistakes
- Adding an intercept while still calling the rule direct proportion.
- Using the invariant y/x for an inverse law instead of xy.
- Selecting only a positive recovery for an unrestricted square law.
- Restoring zero after multiplying away a reciprocal denominator.
- Squaring a principal-root equation without retaining its sign restriction.
- Using the input change factor directly for a square,cube or root law.
- Treating a zero-baseline ratio as an ordinary finite number.
- Assuming the coefficient has no units or that matching finite observations proves a physical law.
What can you explain now?
For a positive-coefficient rule y=k/√x,what happens to y when an allowed positive x is multiplied by9/4?
Compare with the explanation
Multiply the output by2/3.
It is multiplied by2/3,because the denominator root grows by3/2;both inputs must remain in the stated domain.
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Make it stick
Close the reference. Choose the stipulated proportional basis,identify the coefficient from an admitted nonzero basis observation,retain the real input domain,predict exactly,return every recovery candidate through the original law,then justify the change ratio,coefficient dimensions and model limits.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.