Combination

Definition

A combination is a way of selecting a subset of items from a larger set, where the order of the items in the subset does not matter. It is a fundamental concept in probability and combinatorics, and is closely related to the idea of permutations.

5 Must Know Facts For Your Next Test

1. The number of combinations of $k$ items from a set of $n$ items is given by the formula $\binom{n}{k} = \frac{n!}{k!(n-k)!}$, where $n!$ represents the factorial of $n$.
2. Combinations are often used in the Binomial Theorem, which provides a formula for expanding binomial expressions of the form $(x + y)^n$.
3. The Binomial Theorem states that $(x + y)^n = \sum_{k=0}^n \binom{n}{k} x^{n-k} y^k$, where $\binom{n}{k}$ represents the number of combinations of $k$ items from a set of $n$ items.
4. Combinations can be used to calculate the number of subsets of a set, as well as the number of ways to choose a certain number of items from a set without regard to order.
5. Combinations are an important concept in probability theory, as they are used to calculate the number of possible outcomes in certain types of probability problems.

Review Questions

• Explain the relationship between combinations and permutations, and how they differ.
  • Combinations and permutations are both ways of selecting items from a set, but they differ in the importance of order. Combinations focus on the number of unique subsets that can be formed, without regard to the order of the items within the subset. Permutations, on the other hand, consider the order of the items and calculate the number of unique arrangements. The formula for combinations, $\binom{n}{k} = \frac{n!}{k!(n-k)!}$, reflects this distinction by dividing the total number of permutations ($n!$) by the number of ways the $k$ items can be arranged ($k!$) and the number of ways the remaining $n-k$ items can be arranged $(n-k)!$.
• Describe the role of combinations in the Binomial Theorem and its applications.
  • The Binomial Theorem is a formula that allows for the expansion of binomial expressions of the form $(x + y)^n$. The key component of this formula is the binomial coefficient, $\binom{n}{k}$, which represents the number of combinations of $k$ items from a set of $n$ items. This coefficient appears as the coefficient of the $x^{n-k}y^k$ term in the expansion. The Binomial Theorem has numerous applications, including in probability theory, combinatorics, and the development of mathematical identities. Understanding the role of combinations in the Binomial Theorem is crucial for solving a wide range of problems involving binomial expansions.
• Analyze the formula for the number of combinations, $\binom{n}{k} = \frac{n!}{k!(n-k)!}$, and explain how it can be used to solve problems involving the selection of items from a set.
  • The formula for the number of combinations, $\binom{n}{k} = \frac{n!}{k!(n-k)!}$, can be used to solve a variety of problems involving the selection of items from a set. The numerator, $n!$, represents the total number of permutations of the $n$ items. The denominator, $k!(n-k)!$, accounts for the number of ways the $k$ selected items can be arranged and the number of ways the remaining $n-k$ items can be arranged. By dividing the total number of permutations by the number of ways the selected and unselected items can be arranged, the formula gives the number of unique combinations of $k$ items from a set of $n$ items. This formula is fundamental in combinatorics and probability, and can be used to solve problems involving the selection of subsets, the calculation of probabilities, and the enumeration of various mathematical structures.

Related terms

Permutation: A permutation is an arrangement of a set of items in a specific order, where the order of the items matters.

Binomial Coefficient: The binomial coefficient, denoted as $\binom{n}{k}$, represents the number of ways to choose $k$ items from a set of $n$ items, where the order of the items does not matter.

Factorial: The factorial of a non-negative integer $n$, denoted as $n!$, is the product of all positive integers less than or equal to $n$.