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- A-Level Maths

# The Binomial Expansion

**Pascal's Triangle** is used to expand binomial expressions like (a+b)ⁿ. It is created by summing adjacent pairs to find the number beneath this pair, starting from 1. Here are the first five rows:

The (n+ 1)th row of Pascal's triangle gives the coefficients in the expansion of (a+b)ⁿ

### Factorial Notation

Parts of Pascal's triangle can be calculated quickly using **factorial notation, ⁿ Cr **(spoken "n choose r"):

## Expanding (a+b)ⁿ

When n ∈ ℕ (when n is a positive integer) the binomial expansion is in its simplest form:

The general term in an expansion of (a+b)ⁿ is given as:

## Expanding (1+x)ⁿ

If n is a fraction or a negative number, you need to use this form of the binomial expansion:

It is valid when |x| < 1 and when n ∈ℝ

The general term in this expansion is given as:

### x-term Coefficient

When the **x term has a coefficient**, so the binomial is in the form **(1+bx)ⁿ**, treat (bx) as x, and follow the standard expansion for (1+x)ⁿ

The expansion for(1+bx)ⁿis valid for |bx| < 1, or |x| < 1/|b|

### Double Coefficients

If the binomial is in the form **(a+bx)ⁿ**, you have to **take a factor of aⁿ out of each term:**

The expansion for(a+bx)ⁿ, where n is negative or a fraction, is valid when |bx/a| < 1, or |x| < |a/b|

Often, complex expressions can be simplified first by splitting them into

**partial fractions**(see notes sheet on algebraic methods), then by using a binomial expansion.