Question 35874: Exact Finite Probability Variance

Original question and givens

The nonempty finite sample space is S={0, 1, 2, 3}, with event family F=𝒫(S). The total function X:S→R has table {(0, -29), (1, -29), (2, -4), (3, 1022)}. In the displayed order of S, the nonnegative probability weights are [1/(32), 25/(32), 6/(32), 0/(32)], summing exactly to one. Put Z=X. Find Var(Z)=E[(Z−E[Z])²] exactly as a reduced positive-denominator fraction. This is probability-distribution variance, not sample variance with an n−1 denominator.

• G001 — The nonempty finite sample space is S={0, 1, 2, 3}, with event family F=𝒫(S). The total function X:S→R has table {(0, -29), (1, -29), (2, -4), (3, 1022)}. In the displayed order of S, the nonnegative probability weights are [1/(32), 25/(32), 6/(32), 0/(32)], summing exactly to one. Put Z=X. Find Var(Z)=E[(Z−E[Z])²] exactly as a reduced positive-denominator fraction. This is probability-distribution variance, not sample variance with an n−1 denominator.

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-variance

Step-by-step derivation

source roles = [qf-variance-weighted, [F, S, denominator, operation, values, weights]]

derived = [variance, variance, null]

Variance ORIGINAL nonnegative RAW weights sum := sum(1, 25, 6, 0)

Variance ORIGINAL nonnegative RAW weights sum = 32

source roles = [[0, 1, 2, 3], powerset(S), [-29, -29, -4, 1022], [1, 25, 6, 0], 32, null, null]

derived = [Yes]

Variance weight times transformed value i=0 := mul(1, -29)

Variance weight times transformed value i=0 = -29

source roles = [0, 0, -29, 1, null, null]

derived = [-29, 1/32]

Variance weight times transformed value i=1 := mul(25, -29)

Variance weight times transformed value i=1 = -725

source roles = [1, 1, -29, 25, null, null]

derived = [-29, 25/32]

Variance weight times transformed value i=2 := mul(6, -4)

Variance weight times transformed value i=2 = -24

source roles = [2, 2, -4, 6, null, null]

derived = [-4, 3/16]

Variance weight times transformed value i=3 := mul(0, 1022)

Variance weight times transformed value i=3 = 0

source roles = [3, 3, 1022, 0, null, null]

derived = [1022, 0]

Variance mean numerator sum wY := sum(-29, -725, -24, 0)

Variance mean numerator sum wY = -778

Variance exact expectation M over q := div(-778, 32)

Variance exact expectation M over q = -389/16

Variance i=0 Y times q := mul(-29, 32)

Variance i=0 Y times q = -928

Variance i=0 centered integer numerator := sub(-928, -778)

Variance i=0 centered integer numerator = -150

Variance i=0 centered numerator squared := mul(-150, -150)

Variance i=0 centered numerator squared = 22500

Variance i=0 weight times squared numerator := mul(1, 22500)

Variance i=0 weight times squared numerator = 22500

Variance i=1 Y times q := mul(-29, 32)

Variance i=1 Y times q = -928

Variance i=1 centered integer numerator := sub(-928, -778)

Variance i=1 centered integer numerator = -150

Variance i=1 centered numerator squared := mul(-150, -150)

Variance i=1 centered numerator squared = 22500

Variance i=1 weight times squared numerator := mul(25, 22500)

Variance i=1 weight times squared numerator = 562500

Variance i=2 Y times q := mul(-4, 32)

Variance i=2 Y times q = -128

Variance i=2 centered integer numerator := sub(-128, -778)

Variance i=2 centered integer numerator = 650

Variance i=2 centered numerator squared := mul(650, 650)

Variance i=2 centered numerator squared = 422500

Variance i=2 weight times squared numerator := mul(6, 422500)

Variance i=2 weight times squared numerator = 2535000

Variance i=3 Y times q := mul(1022, 32)

Variance i=3 Y times q = 32704

Variance i=3 centered integer numerator := sub(32704, -778)

Variance i=3 centered integer numerator = 33482

Variance i=3 centered numerator squared := mul(33482, 33482)

Variance i=3 centered numerator squared = 1121044324

Variance i=3 weight times squared numerator := mul(0, 1121044324)

Variance i=3 weight times squared numerator = 0

Variance sum ALL weighted squared numerators := sum(22500, 562500, 2535000, 0)

Variance sum ALL weighted squared numerators = 3120000

Variance denominator q squared := mul(32, 32)

Variance denominator q squared = 1024

Variance denominator q cubed binary second factor := mul(1024, 32)

Variance denominator q cubed binary second factor = 32768

Variance probability population centered sum over q cubed := div(3120000, 32768)

Variance probability population centered sum over q cubed = 24375/256

source roles = [[-29, -29, -4, 1022], [1, 25, 6, 0], 32, -389/16, 3120000, 32768]

derived = [24375/256, [-29, -4], No]

source roles = [variance, null, null]

derived = [24375/256]

Reverse checks

• S035 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.
• S034 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.
• S033 · premise_and_result_audit — Recompute Variance probability population centered sum over q cubed with exact rational arithmetic, not rounded decimals.
• S032 · premise_and_result_audit — Recompute Variance denominator q cubed binary second factor with exact rational arithmetic, not rounded decimals.
• S031 · premise_and_result_audit — Recompute Variance denominator q squared with exact rational arithmetic, not rounded decimals.
• S030 · premise_and_result_audit — Recompute Variance sum ALL weighted squared numerators with exact rational arithmetic, not rounded decimals.
• S029 · premise_and_result_audit — Recompute Variance i=3 weight times squared numerator with exact rational arithmetic, not rounded decimals.
• S028 · premise_and_result_audit — Recompute Variance i=3 centered numerator squared with exact rational arithmetic, not rounded decimals.
• S027 · premise_and_result_audit — Recompute Variance i=3 centered integer numerator with exact rational arithmetic, not rounded decimals.
• S026 · premise_and_result_audit — Recompute Variance i=3 Y times q with exact rational arithmetic, not rounded decimals.
• S025 · premise_and_result_audit — Recompute Variance i=2 weight times squared numerator with exact rational arithmetic, not rounded decimals.
• S024 · premise_and_result_audit — Recompute Variance i=2 centered numerator squared with exact rational arithmetic, not rounded decimals.
• S023 · premise_and_result_audit — Recompute Variance i=2 centered integer numerator with exact rational arithmetic, not rounded decimals.
• S022 · premise_and_result_audit — Recompute Variance i=2 Y times q with exact rational arithmetic, not rounded decimals.
• S021 · premise_and_result_audit — Recompute Variance i=1 weight times squared numerator with exact rational arithmetic, not rounded decimals.
• S020 · premise_and_result_audit — Recompute Variance i=1 centered numerator squared with exact rational arithmetic, not rounded decimals.
• S019 · premise_and_result_audit — Recompute Variance i=1 centered integer numerator with exact rational arithmetic, not rounded decimals.
• S018 · premise_and_result_audit — Recompute Variance i=1 Y times q with exact rational arithmetic, not rounded decimals.
• S017 · premise_and_result_audit — Recompute Variance i=0 weight times squared numerator with exact rational arithmetic, not rounded decimals.
• S016 · premise_and_result_audit — Recompute Variance i=0 centered numerator squared with exact rational arithmetic, not rounded decimals.
• S015 · premise_and_result_audit — Recompute Variance i=0 centered integer numerator with exact rational arithmetic, not rounded decimals.
• S014 · premise_and_result_audit — Recompute Variance i=0 Y times q with exact rational arithmetic, not rounded decimals.
• S013 · premise_and_result_audit — Recompute Variance exact expectation M over q with exact rational arithmetic, not rounded decimals.
• S012 · premise_and_result_audit — Recompute Variance mean numerator sum wY with exact rational arithmetic, not rounded decimals.
• S011 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.
• S010 · premise_and_result_audit — Recompute Variance weight times transformed value i=3 with exact rational arithmetic, not rounded decimals.
• S009 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.
• S008 · premise_and_result_audit — Recompute Variance weight times transformed value i=2 with exact rational arithmetic, not rounded decimals.
• S007 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.
• S006 · premise_and_result_audit — Recompute Variance weight times transformed value i=1 with exact rational arithmetic, not rounded decimals.
• S005 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.
• S004 · premise_and_result_audit — Recompute Variance weight times transformed value i=0 with exact rational arithmetic, not rounded decimals.
• S003 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.
• S002 · premise_and_result_audit — Recompute Variance ORIGINAL nonnegative RAW weights sum with exact rational arithmetic, not rounded decimals.
• S001 · premise_and_result_audit — Reverse-audit THIS question’s original operands,roles and necessary premises;reconstruct the next step,without reversing a one-way implication.Numerical/finite-table checking is not formal proof of an arbitrary theorem.Original Q,bilingual sources,MKO,AST,status and history remain preserved.

Result

24375/256

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.