# Properties of Binomial Coefficients

Binomial coefficients, often denoted as C(n, k), are fundamental in combinatorics and algebra, frequently appearing in the binomial theorem and various mathematical contexts. Understanding the properties of binomial coefficients is essential for solving combinatorial problems and working with polynomial expansions. Here are some key properties:

**1. Symmetry:**

- C(n, k) = C(n, n-k)
- This symmetry property arises from the combinatorial interpretation of binomial coefficients. It means that the binomial coefficients are symmetric with respect to the middle value when 'n' is fixed.

**2. Pascal's Triangle:**

- Binomial coefficients can be arranged in Pascal's triangle, a triangular array of numbers. Each number in the triangle is the sum of the two numbers directly above it.
- C(n, k) is found in the nth row and kth position of Pascal's triangle.
- Pascal's triangle makes it easy to compute binomial coefficients and reveals many patterns and relationships.

**3. Recursive Formula:**

- Binomial coefficients can be computed using a recursive formula: C(n, k) = C(n-1, k-1) + C(n-1, k)
- This recursive formula helps in calculating binomial coefficients for large values of 'n' and 'k' efficiently.

**4. Binomial Theorem:**

- The binomial theorem states that for any non-negative integer 'n': ${(a+b)}^{n}$ = Σ [C(n, k) * ${a}^{(n-k)}$ * ${b}^{k}$] for k = 0 to n
- Binomial coefficients determine the coefficients in the expansion of ${(a+b)}^{n}$.

**5. Combinatorial Interpretation:**

- C(n, k) has a combinatorial interpretation. It represents the number of ways to choose 'k' items from a set of 'n' distinct items, ignoring order.

**6. Summation Property:**

- The sum of all binomial coefficients in a given row of Pascal's triangle is equal to ${2}^{n}$.

**7. Algebraic Identity:**

- The binomial coefficient identity states: Σ [C(n, k)] for k = 0 to n = ${2}^{n}$
- This identity is related to the sum of all binomial coefficients in the nth row of Pascal's triangle.

**8. Vandermonde's Identity:**

- Vandermonde's identity is a crucial property of binomial coefficients, given by: Σ [C(m, k) * C(n, r)] for k = 0 to m and r = 0 to n = C(m + n, k + r)
- It is useful for simplifying the products of binomial coefficients.

**9. Sums of Powers:**

- Binomial coefficients appear in sums of powers, such as: Σ [k * C(n, k)] for k = 0 to n = n * ${2}^{(n-1)}$ Σ [${k}^{2}$ * C(n, k)] for k = 0 to n = n * (n+1) * ${2}^{(n-2)}$