# Sum of the rows of Pascal's Triangle.

I've discovered that the sum of each row in Pascal's triangle is $2^n$, where $n$ number of rows. I'm interested why this is so. Rewriting the triangle in terms of C would give us $0C0$ in first row. $1C0$ and $1C1$ in the second, and so on and so forth. However, I still cannot grasp why summing, say, 4C0+4C1+4C2+4c3+4C4=2^4.

## 3 Answers

If you know the binomial theorem, then it's easy: consider $(1+1)^n$.

If you understand $nCk$ in combinatorial terms as the number of subsets of $k$ elements chosen from a universe of $n$ elements, then it's easy because the total number of subsets is $2^n$.

There are various different ways to look at this. Here's one:

Two adjacent numbers in a row get added to get the number in the row below it: $$\begin{array}{cccccccccc} & & 1 & & & & & 8 & & & & 28 & & & & 56 & & & & 70 & & \cdots \\ & & & & & & & & \searrow & & \swarrow \\ 1 & & & & & 9 & & & & 36 & & & & 84 & & & & 126 & & \cdots & & & & \cdots \end{array}$$ That means every number in a row is added into the next row twice: $$\begin{array}{cccccccccc} & & 1 & & & & & 8 & & & & 28 & & & & 56 & & & & 70 & & \cdots \\ & & & & & & \swarrow & & \searrow \\ 1 & & & & & 9 & & & & 36 & & & & 84 & & & & 126 & & \cdots & & & & \cdots \end{array}$$ Since every number is added into the next row twice, the sum of the numbers in the next row is twice as big.

Here's another: In row $9$ (which is the tenth row, since the first row is "row $0$), the entries are. $$\binom 9 0 = 1,\ \binom 9 1 = 9,\ \binom 9 2 = 36,\ \binom 9 3 = 84,\ \binom 9 4 = 126,\ \ldots$$ These are

• the number of subsets of size $0$ of a set of size $9$, and
• the number of subsets of size $1$ of a set of size $9$, and
• the number of subsets of size $2$ of a set of size $9$, and
• the number of subsets of size $3$ of a set of size $9$, and
• and so on.

Their sum is therefore the total number of subsets of a set of size $9$. If you know that that is $2^9$, you've got it.

Use the binomial theorem as:

$$(1+x)^n = {n\choose 0} + {n\choose 1}x^1+ {n\choose 2}x^2+\cdots+{n\choose n}x^n$$ Put $x=1$ to get $$2^n= {n\choose 0} + {n\choose 1}+ {n\choose 2}+\cdots+{n\choose n}.$$