THE BINOMIAL EXPANSION - Mathematics with Mr Walters
THE BINOMIAL EXPANSION
Reminder:
PASCAL’S TRIANGLE
[pic]
The row beginning 1 6 …. gives the coefficients of the expansion of (1 + x)6 in increasing powers of x
Expressions such as 1 + x, 2 − 5x, a + bx are called binomials as they consist of just two terms.
Expressions with many terms such as 7 + 3x − x2 + 5x3 are called polynomials
Example
Expand (1 + 2x)4
[pic]
Note: the power of x increases by 1 as you progress through the series
[pic]
EXPANSION OF (a + b)n
By multiplying out brackets it is easily established that:
[pic]
Note:
• Pascal’s Triangle is used for successive coefficients
• the powers of a decrease by one each time
• the powers of b increase by one each time
Example
Expand (2x − 5)4 in descending powers of x
[pic]
THE BINOMIAL THEOREM
Expanding (a + b)n using Pascal’s Triangle is all very well if the value of n is not too large – but it is a real pain if n gets beyond 10 say!
Using the 4th row of Pascal’s Triangle:
1 4 6 4 1
It can be shown that:
[pic]
Similarly the 5th row
1 5 10 10 5 1
can be shown to come from:
[pic]
So, for the power n, the coefficients in order are:
[pic]
[pic]
Therefore,
Example
Find the first 4 terms in the expansion of
(1 + x)12
[pic]
It also follows that:
If n is a positive integer, the expansion is finite and exact
Example [pic]
If n is negative or a fraction, this is not generally the case.
Example (negative index)
Expand [pic]
[pic]
This expansion is infinite (since none of the numbers in the numerator are ever zero, so it just goes on and on and …)
If the size of x (|x| = “mod x”) is 1 or more then the series will diverge. It will converge if |x| < 1 (this series is convergent if |x| < 1)
Example (fractional index)
Find the expansion of [pic] up to the term in x3
[pic]This is valid for |2x| ................
................
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