Binomial Coefficient Calculator
Solve binomial coefficient problems step-by-step with our free calculator. See formulas, worked examples, and clear explanations.
Formula
C(n, k) = n! / (k! * (n - k)!)
The binomial coefficient counts the number of ways to choose k items from n distinct items without regard to order. n! (n factorial) is the product of all integers from 1 to n. The denominator k! * (n-k)! removes the overcounting from internal ordering of chosen and unchosen items.
Worked Examples
Example 1: Computing C(10, 3)
Problem: How many ways can you choose 3 items from 10 distinct items?
Solution: C(10, 3) = 10! / (3! * 7!)\n= (10 * 9 * 8) / (3 * 2 * 1)\n= 720 / 6\n= 120\nVerification: C(10, 7) = 120 (symmetry property confirms)
Result: C(10, 3) = 120 ways
Example 2: Lottery Probability
Problem: In a lottery choosing 6 numbers from 49, how many possible combinations exist?
Solution: C(49, 6) = 49! / (6! * 43!)\n= (49 * 48 * 47 * 46 * 45 * 44) / (6 * 5 * 4 * 3 * 2 * 1)\n= 10068347520 / 720\n= 13,983,816\nProbability of winning = 1 / 13,983,816
Result: C(49, 6) = 13,983,816 combinations
Frequently Asked Questions
What is a binomial coefficient and what does C(n,k) represent?
A binomial coefficient, written as C(n,k) or 'n choose k', represents the number of ways to choose k items from a set of n distinct items, without regard to the order of selection. It is one of the most fundamental concepts in combinatorics and probability theory. The notation comes from its role in the binomial theorem, where it appears as the coefficient of x^k in the expansion of (1+x)^n. For example, C(5,2) = 10 means there are exactly 10 ways to select 2 items from 5. The formula is C(n,k) = n! / (k! * (n-k)!), where ! denotes the factorial function.
How is the binomial coefficient calculated using the factorial formula?
The binomial coefficient C(n,k) equals n! divided by the product of k! and (n-k)!. The factorial function n! means the product of all positive integers from 1 to n. For example, C(10,3) = 10! / (3! * 7!) = 3628800 / (6 * 5040) = 3628800 / 30240 = 120. In practice, direct factorial computation is avoided for large numbers because factorials grow extremely fast (20! already exceeds 2 quintillion). Instead, an iterative multiplication approach is used where the computation is done step by step with intermediate cancellation: C(10,3) = (10 * 9 * 8) / (3 * 2 * 1) = 120.
What is the symmetry property of binomial coefficients?
The symmetry property states that C(n,k) = C(n, n-k), meaning choosing k items from n is the same as choosing which n-k items to leave out. For example, C(10,3) = C(10,7) = 120. This makes intuitive sense because every way of choosing 3 items simultaneously determines which 7 items are not chosen, creating a one-to-one correspondence between the two sets of selections. This property is useful computationally because you can always compute C(n,k) using the smaller of k or n-k, reducing the number of multiplications needed. It is also visually apparent in Pascal's triangle, which is symmetric about its center vertical axis.
What is Pascal's triangle and how does it relate to binomial coefficients?
Pascal's triangle is a triangular arrangement of numbers where each entry is a binomial coefficient. Row n contains the values C(n,0), C(n,1), ..., C(n,n). Each entry equals the sum of the two entries directly above it: C(n,k) = C(n-1,k-1) + C(n-1,k). This recursive property provides an elegant way to compute binomial coefficients without using factorials. The triangle begins with row 0 containing just 1, row 1 containing 1,1, row 2 containing 1,2,1, and so on. Pascal's triangle contains many remarkable patterns including the Fibonacci sequence (along diagonals), powers of 2 (row sums), and powers of 11 (reading rows as digits).
How are binomial coefficients used in probability calculations?
Binomial coefficients are essential for computing probabilities in the binomial distribution, which models the number of successes in n independent trials each with probability p. The probability of exactly k successes is P(X=k) = C(n,k) * p^k * (1-p)^(n-k). For a fair coin (p=0.5), the probability simplifies to C(n,k)/2^n. For example, the probability of getting exactly 3 heads in 10 fair coin flips is C(10,3)/1024 = 120/1024 = 0.1172 or about 11.72%. This formula is used extensively in quality control, clinical trials, genetic studies, polling, and any scenario involving repeated independent binary outcomes.
What is the binomial theorem and how does it use binomial coefficients?
The binomial theorem states that (a + b)^n = sum from k=0 to n of C(n,k) * a^(n-k) * b^k. This powerful formula expands any binomial expression raised to a positive integer power. For example, (x + y)^3 = C(3,0)x^3 + C(3,1)x^2y + C(3,2)xy^2 + C(3,3)y^3 = x^3 + 3x^2y + 3xy^2 + y^3. The binomial coefficients appear as the multipliers of each term. Setting a = b = 1 gives the identity that the sum of all entries in row n of Pascal's triangle equals 2^n. The theorem extends to negative and fractional exponents through the generalized binomial series discovered by Newton.