Combinatorics MOC

Binomial expansion

The binomial expansion states that num

where the so-called binomial coëfficients are given by

and is the number of ways to choose elements of a set of size . See also Generalized binomial coëfficient.

Properties

\begin{align*} {n \choose k} = {n \choose n-k} \end{align*}

^P2 3. $$ \begin{align*} {m+n\choose k} = \sum_{j=0}^k {m \choose j}{n \choose k-j} \end{align*}

^P4 5. $$ \begin{align*} \sum_{m=k}^n {m \choose k} = {n+1 \choose k+1} \end{align*}

You can't use 'macro parameter character #' in math mode^P5 > [!check]- Proof of 1–3, 5 > > Clearly choosing $k$ elements from a set of size $n$ is the same as choosing $n-k$ elements to be excluded, proving [[#^p1|^P1]]. > > Consider choosing a team of size $k$ from a set of $n$ people, where one member of the team is the captain. > One can either first choose a captain and then the rest of the team (LHS), > or the team and thence the captain (RHS), > proving [[#^p2|^P2]]. > > Consider a set of $m$ red marbles and $n$ blue marbles. > The number of arbitrary choices of $k$ marbles is the LHS, > but this is the same as every possible way of choosing $j$ red marbles and $k-j$ blue marbles (RHS). > This proves [[#^p3|^P3]]. > > A proof of [[#^p4|^P4]] is missing, but [[#^p5|^P5]] follows directly for $j=k$. > <span class="QED"/> > [!missing]- Proof of 4 > > #missing/proof # --- #state/develop | #lang/en | #SemBr