Question 430: Compare the Whole Binomial Sum with an Exact Integer Power

Original question and givens

Use natural exponent n=4, exact commuting integers x=1, y=-2. Give (n, x, y, power, expansion_sum, equal).

• G001 — Use natural exponent n=4, exact commuting integers x=1, y=-2. 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=4≥0; multiplication commutes

group the finite product by the number of y choices

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

x+y = -1

(x+y)^n: (-1)^4

(x+y)^n = 1

choose 0 positions out of 4

C(4,0)=24/(1·24)=1

x^4: (1)^4

x^4 = 1

y^0: (-2)^0

y^0 = 1

C·x power (k=0) := mul(1, 1)

C·x power (k=0) = 1

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

term[0] = 1

choose 1 positions out of 4

C(4,1)=24/(1·6)=4

x^3: (1)^3

x^3 = 1

y^1: (-2)^1

y^1 = -2

C·x power (k=1) := mul(4, 1)

C·x power (k=1) = 4

term[1] := mul(4, -2)

term[1] = -8

choose 2 positions out of 4

C(4,2)=24/(2·2)=6

x^2: (1)^2

x^2 = 1

y^2: (-2)^2

y^2 = 4

C·x power (k=2) := mul(6, 1)

C·x power (k=2) = 6

term[2] := mul(6, 4)

term[2] = 24

choose 3 positions out of 4

C(4,3)=24/(6·1)=4

x^1: (1)^1

x^1 = 1

y^3: (-2)^3

y^3 = -8

C·x power (k=3) := mul(4, 1)

C·x power (k=3) = 4

term[3] := mul(4, -8)

term[3] = -32

choose 4 positions out of 4

C(4,4)=24/(24·1)=1

x^0: (1)^0

x^0 = 1

y^4: (-2)^4

y^4 = 16

C·x power (k=4) := mul(1, 1)

C·x power (k=4) = 1

term[4] := mul(1, 16)

term[4] = 16

expanded sum := sum(1, -8, 24, -32, 16)

expanded sum = 1

direct=1; expanded=1

identity holds: Yes

Reverse checks

• S030 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S029 · premise_and_result_audit — Recompute expanded sum with exact rational arithmetic, not rounded decimals.
• S028 · premise_and_result_audit — Recompute term[4] with exact rational arithmetic, not rounded decimals.
• S027 · premise_and_result_audit — Recompute C·x power (k=4) with exact rational arithmetic, not rounded decimals.
• S026 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S025 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S024 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S023 · premise_and_result_audit — Recompute term[3] with exact rational arithmetic, not rounded decimals.
• S022 · premise_and_result_audit — Recompute C·x power (k=3) with exact rational arithmetic, not rounded decimals.
• S021 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S020 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S019 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• S018 · premise_and_result_audit — Recompute term[2] with exact rational arithmetic, not rounded decimals.
• S017 · premise_and_result_audit — Recompute C·x power (k=2) with exact rational arithmetic, not rounded decimals.
• S016 · premise_and_result_audit — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• 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 — Count coefficients in the integers first; do not invert possibly nonunit factorials in a general coefficient ring.
• 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

(4, 1, -2, 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.