# Binomial Expansion

• January 10th 2009, 02:59 AM
Simplicity
Binomial Expansion
Can someone remind me which Binomial Expansion formula is used to expand:

$\frac{1}{1+x^2} = 1 + (-1)x^2 + \frac{(-1)(-2)}{2!}(x^2)^2 + \frac{(-1)(-2)(-3)}{3!}(x^2)^3+...$

• January 10th 2009, 03:43 AM
mr fantastic
Quote:

Originally Posted by Air
Can someone remind me which Binomial Expansion formula is used to expand:

$\frac{1}{1+x^2} = 1 + (-1)x^2 + \frac{(-1)(-2)}{2!}(x^2)^2 + \frac{(-1)(-2)(-3)}{3!}(x^2)^3+...$

You can do it much easier using the sum of an infinite geometric series formula:

$1 + r + r^2 + \, .... = \frac{1}{1 - r}$

(provided |r| < 1).

In your case $r = -x^2$.
• January 10th 2009, 03:53 AM
HallsofIvy
If you really want to do it using the (generalized) binomial expansion, do it as $(a+ 1)^n$ with $a= x^2$ and n= -1.