Question 8120: Nonzero-Slope Real Affine Inverse and Two Rational Round Trips

Original question and givens

Declare f:R→R by f(x)=a*x+b with a=-5≠0, b=7; ordered rational inputs y=[7/3, 20/4] have nonzero original denominators. Give ([inverse slope, inverse constant], recovered x values, f(x) values, do both return their original y?). Use reduced positive-denominator fractions, including /1.

• G001 — Declare f:R→R by f(x)=a*x+b with a=-5≠0, b=7; ordered rational inputs y=[7/3, 20/4] have nonzero original denominators. Give ([inverse slope, inverse constant], recovered x values, f(x) values, do both return their original y?). Use reduced positive-denominator fractions, including /1.

Definition and scope

Use the original question domain, assumptions and cited MKO definitions. This item has second-context mathematical and bilingual review, with the model, answer and every arithmetic step independently recomputed from the original parameters; the source question remains a candidate.

• mko-inverse-function

Step-by-step derivation

a=-5;b=7;raw ys=[[7, 3], [20, 4]]

normalized ys=[7/3, 5/1];a!=0

inverse slope := div(1, -5)

inverse slope = -1/5

negative intercept := sub(0, 7)

negative intercept = -7

inverse intercept := div(-7, -5)

inverse intercept = 7/5

g(y)=(y-b)/a;f(x)=a*x+b;a!=0

g(f(x))=((a*x+b)-b)/a=x;f(g(y))=a*((y-b)/a)+b=y

query0 y-b := sub(7/3, 7)

query0 y-b = -14/3

query0 recovered x := div(-14/3, -5)

query0 recovered x = 14/15

query0 a*x := mul(-5, 14/15)

query0 a*x = -14/3

query0 f(x) := sum(-14/3, 7)

query0 f(x) = 7/3

query0:y=7/3;x=14/15

f(x)=7/3;matches=Yes

query1 y-b := sub(5, 7)

query1 y-b = -2

query1 recovered x := div(-2, -5)

query1 recovered x = 2/5

query1 a*x := mul(-5, 2/5)

query1 a*x = -2

query1 f(x) := sum(-2, 7)

query1 f(x) = 5

query1:y=5/1;x=2/5

f(x)=5/1;matches=Yes

Reverse checks

• S015 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
• S014 · premise_and_result_audit — Recompute query1 f(x) with exact rational arithmetic, not rounded decimals.
• S013 · premise_and_result_audit — Recompute query1 a*x with exact rational arithmetic, not rounded decimals.
• S012 · premise_and_result_audit — Recompute query1 recovered x with exact rational arithmetic, not rounded decimals.
• S011 · premise_and_result_audit — Recompute query1 y-b with exact rational arithmetic, not rounded decimals.
• S010 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
• S009 · premise_and_result_audit — Recompute query0 f(x) with exact rational arithmetic, not rounded decimals.
• S008 · premise_and_result_audit — Recompute query0 a*x with exact rational arithmetic, not rounded decimals.
• S007 · premise_and_result_audit — Recompute query0 recovered x with exact rational arithmetic, not rounded decimals.
• S006 · premise_and_result_audit — Recompute query0 y-b with exact rational arithmetic, not rounded decimals.
• S005 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
• S004 · premise_and_result_audit — Recompute inverse intercept with exact rational arithmetic, not rounded decimals.
• S003 · premise_and_result_audit — Recompute negative intercept with exact rational arithmetic, not rounded decimals.
• S002 · premise_and_result_audit — Recompute inverse slope with exact rational arithmetic, not rounded decimals.
• S001 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.

Result

([-1/5, 7/5], [14/15, 2/5], [7/3, 5/1], Yes)

The full substitutions and premise checks give the original answer; source identity,format and status are unchanged.

This question review and source binding

Review date: 2026-10-26

Original questions, answers, step expressions, numeric ASTs and historical records are retained. Source candidate labels remain historical and are not overwritten as formal proof.