The binomial theorem

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The binomial theorem appears in many proofs across mathematics and mathematical statistics. In this post, I will walk through a proof of this theorem.

Introduction

The Binomial Theorem is a basic theorem in mathematics that states that a binomial expression of the form $(x+y)^n$ can be represented as a summation involving the binomial coefficient:

$(x+y)^n = \sum_{k=0}^n {n \choose k} x^ky^{n-k}$

This statement always appeared mysterious to me. What is the intuition for the summation? Moreover, why does the binomial coefficient, “$n$ choose $k$”, appear? In this post I will provide a proof that helped me better intuit this theorem.

Proof

$(a + b)(c + d)(e + f)$

We can apply the distributive property as follows:

$(a + b)(c + d)(e + f) = a(c+d)(e+f) + b(c+d)(e+f)$

Notice that the first and second terms are two separate polynomials involving $a$ and $b$ where the first only involves $a$, but not $b$, whereas the second polynomial only involves $b$, but not $a$. In fact, for each binomial factor (i.e. for each of $(a + b)$, $(c + d)$, and $(e + f)$), only one of the terms of the binomial will appear in a given term of the fully expanded polynomial.

\begin{align*}(a + b)(c + d)(e + f) &= a(c+d)(e+f) + b(c+d)(e+f) \\ &= a(c(e+f) + d(e+f)) + b(c(e+f) + d(e+f)) \\ &= ace + acf + ade + adf + bce + bcf + bde + bdf\end{align*}

To form a term in the fully expanded polynomial, we imagine the process of iterating over each binomial factor and choosing one of the two terms to include. For example, from $(a+b)$, we choose either $a$ or $b$ to include in the term, but never both. This is because of how the distributive property works: as we expanded the expression, we separated the two terms in each binomial factor so that they never could appear in the same term of the expansion. This process is depicted in the following diagram:

We also see that every combination of terms from each binomial factor will be used to form a term in the expanded polynomial. Again, this occurs from the process of carrying out the distributive property repeatedly when carrying out the expansion: everytime we perform the distributive property, we create two batches of terms in the expansion that will include either the first or second term from that binomial factor. Each batch is created in a split in the tree diagram above.

With this insight, let’s look at the following polynomial:

\begin{align*}(x + y)^3 &= (x + y)(x + y)(x + y) \\ &= x(x+y)(x+y) + y(x+y)(x+y) \\ &= x(x(x+y) + y(x+y)) + y(x(x+y) + y(x+y)) \\ &= xxx + xyx + yxx + yyx + xxy + xyy + yxy + yyy \\ &= x^3 + 3x^2y + 3y^2x + y^3\end{align*}

Let’s say we’re interested in all terms in the expanded polynomial that have $k$ of the $x$ terms. By the previous observation, any term that has $k$ of the $x$ terms must have $n − k$ of the $y$ terms because we only pick a single term from each of the original binomial factors. Now, how many of the terms in the expanded polynomial will have $k$ of the $x$ terms and $n-k$ of the $y$ terms? Recall that every combination of ways of picking $k$ of the $x$ terms from the binomial factors will result in a term in the expansion of the form $x^ky^{n-k}$. We can think of this as computing all possible ways of choosing $k$ $x$ terms from the $n$ binomial factors. Thus, there will be ${n \choose k}$ such terms.

Finally, there are terms in the polynomial with $k$ $x$ terms for every value of $k$ between $0$ and $n$. Again, this follows from from the fact that every combination of terms from each of the binomial factor will be used to form a term in the expanded polynomial, and thus, there must be at least one term with $k$ of the $x$ terms and $n-k$ of the $y$ terms. This final observation leads to the Binomial Theorem:

$(x+y)^n = \sum_{k=0}^n {n \choose k} x^ky^{n-k}$

$\square$

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