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Question 311: Cross-Multiplication Check of a Rational Solution

On this page
  1. Original question and givens
  2. Definition and scope
  3. Step-by-step derivation
  4. Reverse checks
  5. Result
  6. This question review and source binding

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P1000-0311 · q-20260927-linear-equation-solution-verification-0011

Original question and givens

Over the rational numbers, consider -3x+(7)=0 and the candidate x=7/3. Is the candidate a solution?

  • G001 — Over the rational numbers, consider -3x+(7)=0 and the candidate x=7/3. Is the candidate a solution?

Definition and scope

Use the rational domain. A candidate p/q needs q≠0; a zero residual is equivalent to the cross-multiplied identity.

Step-by-step derivation

S001 · implies

Before

a=-3, b=7, x∈Q

After

equation: -3x+(7)=0

Conditions: none

Read the linear coefficient, constant and rational domain explicitly; a name alone does not establish nondegeneracy.

Step identifiers and dependencies

Rule: read_coefficients · Uses: G001

S002 · calculation

Before

candidate x := div(7, 3)

After

candidate x = 7/3

Conditions: none

Read the candidate as an exact rational; its denominator must be nonzero.

Step identifiers and dependencies

Rule: exact_rational_arithmetic · Uses: G001, S001

S003 · calculation

Before

a·x := mul(-3, 7/3)

After

a·x = -7

Conditions: none

Use the original coefficient in the linear term.

Step identifiers and dependencies

Rule: exact_rational_arithmetic · Uses: G001, S002

S004 · calculation

Before

a·x+b := sum(-7, 7)

After

a·x+b = 0

Conditions: none

The candidate solves the original equation exactly when the residual is0.

Step identifiers and dependencies

Rule: exact_rational_arithmetic · Uses: G001, S003

S005 · calculation

Before

a·p := mul(-3, 7)

After

a·p = -21

Conditions: none

Independently form the first cross-multiplied integer term.

Step identifiers and dependencies

Rule: exact_rational_arithmetic · Uses: G001, S004

S006 · calculation

Before

b·q := mul(7, 3)

After

b·q = 21

Conditions: none

Multiply the constant by the same nonzero denominator.

Step identifiers and dependencies

Rule: exact_rational_arithmetic · Uses: G001, S005

S007 · calculation

Before

a·p+b·q := sum(-21, 21)

After

a·p+b·q = 0

Conditions: none

Nonzero q makes this equality equivalent to a zero original residual.

Step identifiers and dependencies

Rule: exact_rational_arithmetic · Uses: G001, S006

S008 · implies

Before

residual=0; cross=0

After

candidate solves equation: Yes

Conditions: none

The two exact checks agree on the candidate's validity.

Step identifiers and dependencies

Rule: substitution_verdict · Uses: G001, S007

Reverse checks

  • S008 · premise_and_result_audit — Substitute into the original equation; retain every nonzero condition before division.
  • S007 · premise_and_result_audit — Recompute a·p+b·q with exact rational arithmetic, not rounded decimals.
  • S006 · premise_and_result_audit — Recompute b·q with exact rational arithmetic, not rounded decimals.
  • S005 · premise_and_result_audit — Recompute a·p with exact rational arithmetic, not rounded decimals.
  • S004 · premise_and_result_audit — Recompute a·x+b with exact rational arithmetic, not rounded decimals.
  • S003 · premise_and_result_audit — Recompute a·x with exact rational arithmetic, not rounded decimals.
  • S002 · premise_and_result_audit — Recompute candidate x with exact rational arithmetic, not rounded decimals.
  • S001 · premise_and_result_audit — Substitute into the original equation; retain every nonzero condition before division.

Result

Yes

The full substitutions and premise audits recover the original answer, retaining its field order and verdict meanings.

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.

Source hashes and review receipts
  • input commit: 0f7be61e2c22317a4075561dc822c87d881a50db
  • original question SHA-256: a7a75505b52bbba8b00df4867dab7082dcc92eead25ccdca43df76da0b20d799
  • candidate record SHA-256: 7e0d66b95af023dc4350274ec6aa9c61a9ba7d3253001e09691bef98e9ebaf6f
  • semantic review receipt SHA-256: 6c2c6d46aded7582a11ac41a8c0b3bdde2669a53bdb6e796eea2039994a02772
  • question-first witness SHA-256: 77e43898df9efdbf8094ad81fe4ab91cf4b4bd82f481eb9b4699b73cb4da612b
  • Lean: NOT_RUN · human review: NOT_PERFORMED
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