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IB DP Maths AA SL Study Notes

1.4.1 Basics of Binomial Expansion

Pascal's Triangle

Pascal's Triangle, named after the French mathematician Blaise Pascal, is a triangular array of numbers with remarkable properties and applications in various areas of mathematics, particularly in binomial expansions. For more advanced applications, see direct and indirect proofs.

Constructing Pascal's Triangle

  • Step 1: Begin with a solitary "1" at the apex.
  • Step 2: The second row is also a lone "1."
  • Step 3: Every subsequent row initiates and concludes with "1."
  • Step 4: Each interior number is the sum of the two numbers directly above it. Understanding the introduction to sigma notation can further help with series and sums.

Properties and Applications in Binomial Expansion

  • Row Number: The row number (beginning from 0) indicates the power to which a binomial is elevated.
  • Coefficients: The numbers in the row provide the coefficients in the expanded binomial expression.

The sum of the numbers in the nth row is 2n, and the sum of rows 0 through n is 2(n + 1) - 1. The generating function of the nth row is (x + 1)n.

Binomial Coefficients

Binomial coefficients, often referred to as "combinations" or "n choose k," are the numerical coefficients used in expressing polynomial powers and expansions, particularly in the binomial theorem. This concept is also important when working with binomial distribution.

Notation and Formula

  • Symbol: Binomial coefficients are typically denoted by "n choose k" or "C(n, k)".
  • Formula: C(n, k) = n! / (k!(n-k)!), where "!" denotes factorial, meaning n! = n * (n-1) * (n-2) * ... * 3 * 2 * 1.

Properties and Applications

  • Symmetry Property: C(n, k) = C(n, n-k).
  • Addition Property: C(n, k) + C(n, k+1) = C(n+1, k+1).

Binomial coefficients are pivotal in combinatorics, aiding in calculating combinations and permutations, and they play a crucial role in the binomial expansion formula. Additionally, understanding logarithmic equations can help simplify complex calculations.

Binomial Expansion Formula

The binomial expansion formula is a fundamental theorem in algebra that describes the expansion of powers of a binomial—expressions of the form (a + b)n. This is particularly useful when dealing with exponential equations.

The Formula

(a + b)n = C(n, 0)an b0 + C(n, 1)a(n-1)b1 + C(n, 2)a(n-2)b2 + ... + C(n, n)a0 bn

Key Points and Applications

  • Exponents: The exponents of a decrease, while those of b increase, always summing up to n.
  • Coefficients: The coefficients are derived from Pascal's Triangle.

Example Question

Let's expand (x + y)4 using the binomial expansion formula.

Using the formula and the 5th row of Pascal's Triangle (1 4 6 4 1), we get:

(x + y)4 = 1 * x4y0 + 4 * x3y1 + 6 * x2y2 + 4 * x1y3 + 1 * x0y4

So, (x + y)4 = x4 + 4x3y + 6x2y2 + 4xy3 + y4

Solving the Example

In the expansion, each term is formed by multiplying the corresponding coefficient from Pascal's Triangle by the terms x and y raised to the appropriate powers. The powers of x decrease from 4 to 0, while the powers of y increase from 0 to 4. The sum of the exponents in each term is always equal to the power to which the binomial is raised. For related topics, see introduction to sigma notation.


The traditional Binomial Theorem is specifically designed for binomials, which are expressions with two terms. However, the concept can be extended to polynomials with more than two terms using the Multinomial Theorem. The Multinomial Theorem allows for the expansion of a polynomial expression (a + b + c + ...)n and involves multinomial coefficients, which are generalisations of binomial coefficients. The coefficients and terms in the expansion become more complex as they involve multiple variables and their respective powers. While the Binomial Theorem provides a straightforward approach for two-term expressions, the Multinomial Theorem caters to the expansion of expressions with three or more terms.

The Binomial Theorem and the Fibonacci Sequence are related through a fascinating connection with Pascal's Triangle. If you sum up the entries of the diagonal elements starting from the leftmost element of each row in Pascal's Triangle, you obtain the Fibonacci Sequence. Mathematically, this can be expressed as F(n) = C(n, 0) + C(n-1, 1) + C(n-2, 2) + ... + C(1, n-1) + C(0, n), where F(n) is the nth Fibonacci number and C(n, k) represents the binomial coefficient. This relationship beautifully illustrates how two seemingly distinct mathematical concepts, the Binomial Theorem (via Pascal's Triangle) and the Fibonacci Sequence, intersect and enrich our understanding of numerical patterns and structures.

The Binomial Theorem can be employed to swiftly calculate powers of numbers, especially when dealing with expressions in the form of (a + b)n. By using the theorem, you can expand the expression without having to multiply it out the long way. For instance, to calculate 1.0510 quickly, you can rewrite 1.05 as (1 + 0.05) and use the Binomial Theorem to expand (1 + 0.05)10. The expansion will involve terms like C(10, k) * 1(10-k) * 0.05k, which are significantly easier and quicker to calculate than performing the multiplication 1.05 * 1.05 * ... ten times. This method is particularly useful for approximating powers of numbers close to a base number.

The Binomial Theorem is deeply intertwined with combinatorics and probability theory, particularly in calculating combinations and determining probabilities in binomial experiments. In combinatorics, the coefficients in the binomial expansion, also known as binomial coefficients, represent the number of ways to choose k items from a set of n (denoted as "n choose k" or C(n, k)). In probability theory, especially in binomial distributions, these coefficients are used to find the probability of obtaining exactly k successes in n independent Bernoulli trials. The theorem provides a systematic way to expand the powers of binomial expressions, which is crucial in determining specific terms in probability distributions and combinatorial problems, thereby forming a bridge between algebraic expansions and discrete mathematics.

In the binomial coefficient, denoted as C(n, k) or "n choose k", n is always larger than or equal to k because it represents the total number of items to choose from, while k represents the number of items to choose. Mathematically, the binomial coefficient is defined as C(n, k) = n! / (k!(n-k)!). If k were larger than n, the factorial in the denominator (n-k)! would involve taking the factorial of a negative number, which is undefined. Furthermore, from a combinatorial perspective, it is impossible to choose k items from a set of n if k > n, as there aren’t enough items to choose from. Thus, ensuring n ≥ k maintains the mathematical and logical validity of the binomial coefficient.

Practice Questions

Expand and simplify the expression (2x - 3y)^5 using the binomial expansion formula.

The binomial expansion formula is given by (a + b)n = C(n, 0)an b0 + C(n, 1)a(n-1)b1 + ... + C(n, n)a0 bn. Using this formula and the 6th row of Pascal's Triangle (1 5 10 10 5 1), we can expand (2x - 3y)5 as follows:

(2x - 3y)5 = 1 * (2x)5(-3y)0 + 5 * (2x)4(-3y)1 + 10 * (2x)3(-3y)2 + 10 * (2x)2(-3y)3 + 5 * (2x)1(-3y)4 + 1 * (2x)0(-3y)5

Calculating the powers and coefficients, we get:

= 32x5 + (-480)x4y + 2880x3y2 + (-6480)x2y3 + 4860xy4 - 729y5

Thus, the expanded and simplified form of (2x - 3y)5 is 32x5 - 480x4y + 2880x3y2 - 6480x2y3 + 4860xy4 - 729y5.

Find the 7th term in the expansion of (a + b)^12.

To find the 7th term in the expansion of (a + b)12, we can use the term formula for binomial expansion, which is given by T(k+1) = C(n, k) * a(n-k) * bk, where n is the power to which the binomial is raised, and k is the position of the term minus 1.

In this case, n = 12 and k = 6 (since we want the 7th term). Using the formula for binomial coefficients C(n, k) = n! / (k!(n-k)!), we find:

C(12, 6) = 12! / (6!(12-6)!) = 924

Therefore, the 7th term T(7) is:

T(7) = C(12, 6) * a(12-6) * b6 = 924 * a6 * b6

So, the 7th term in the expansion of (a + b)12 is 924a6b6.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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