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Binomial coefficients and Pascal's triangle

Identities, the binomial theorem, and combinatorial proofs.

C(n, k) counts k-subsets, is the coefficient in (a + b)ⁿ, and satisfies Pascal's rule C(n, k) = C(n−1, k−1) + C(n−1, k). Picture it: Pascal's triangle, each entry the sum of the two above. Think it: a combinatorial proof shows two formulas count the same set, no algebra needed.

工作范例 · expand (a + b)^6

Algebra · Expanding

Expand (a + b)^6

\left(a + b\right)^{6}

一步

  1. \left(a + b\right)^{6}

    Start from the expression.

  2. a^{6} + 6 a^{5} b + 15 a^{4} b^{2} + 20 a^{3} b^{3} + 15 a^{2} b^{4} + 6 a b^{5} + b^{6}

    Distribute (multiply out) the products.

发送答案
a^{6} + 6 a^{5} b + 15 a^{4} b^{2} + 20 a^{3} b^{3} + 15 a^{2} b^{4} + 6 a b^{5} + b^{6}

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如何: Binomial coefficients and Pascal's triangle

  1. Start from the expression.
  2. Distribute (multiply out) the products.

人们对人们提出的问题

Permutation or combination?

Ask whether order matters. A lock code is a permutation (order matters); a hand of cards is a combination (it does not).

What is a graph in this sense?

Dots (vertices) joined by lines (edges), not a plot. Road maps, social networks and molecules are graphs; questions like "is there a route" and "how few colours" are graph theory.

更多 Combinatorics & Graph Theory

The counting principlesPigeonhole principle and inclusion, exclusionRecurrences and generating functionsGraphs: vertices, edges, degreesPaths, cycles, trees, Euler and HamiltonColouring and planar graphs