Question 427: Compare the Whole Binomial Sum with an Exact Integer Power

Original question and givens

Use natural exponent n=1, exact commuting integers x=-2, y=1. Give (n, x, y, power, expansion_sum, equal).

• G001 — Use natural exponent n=1, exact commuting integers x=-2, y=1. Give (n, x, y, power, expansion_sum, equal).

Definition and scope

n is a nonnegative integer and x,y commute. Sum every term C(n,k)x^(n−k)y^k; the zeroth power is 1.

• mko-binomial-theorem

Step-by-step derivation

n=1≥0; multiplication commutes

group the finite product by the number of y choices

x+y := sum(-2, 1)

x+y = -1

(x+y)^n: (-1)^1

(x+y)^n = -1

choose 0 positions out of 1

C(1,0)=1/(1·1)=1

x^1: (-2)^1

x^1 = -2

y^0: (1)^0

y^0 = 1

C·x power (k=0) := mul(1, -2)

C·x power (k=0) = -2

term[0] := mul(-2, 1)

term[0] = -2

choose 1 positions out of 1

C(1,1)=1/(1·1)=1

x^0: (-2)^0

x^0 = 1

y^1: (1)^1

y^1 = 1

C·x power (k=1) := mul(1, 1)

C·x power (k=1) = 1

term[1] := mul(1, 1)

term[1] = 1

expanded sum := sum(-2, 1)

expanded sum = -1

direct=-1; expanded=-1

identity holds: Yes

Reverse checks

• S015 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S014 · premise_and_result_audit — Recompute expanded sum with exact rational arithmetic, not rounded decimals.
• S013 · premise_and_result_audit — Recompute term[1] with exact rational arithmetic, not rounded decimals.
• S012 · premise_and_result_audit — Recompute C·x power (k=1) with exact rational arithmetic, not rounded decimals.
• S011 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S010 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S009 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S008 · premise_and_result_audit — Recompute term[0] with exact rational arithmetic, not rounded decimals.
• S007 · premise_and_result_audit — Recompute C·x power (k=0) with exact rational arithmetic, not rounded decimals.
• S006 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S005 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S004 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S003 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S002 · premise_and_result_audit — Recompute x+y with exact rational arithmetic, not rounded decimals.
• S001 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.

Result

(1, -2, 1, -1, -1, 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.