# Approximate formula

1. Jul 4, 2005

### Yegor

There exist such a formula for large x:
$$\arctan(x)\approx \pi/2-1/x+1/3x^3...$$
I can't understand how it is derived. I tried to get it from Taylor series (for x -> infinity) and understood that here is something different. Can someone help me?
Thank you

2. Jul 4, 2005

### qbert

have you tried arctan(1/x) for x->0?

and note:
arctan(1/x) = Pi/2 - arctan(x)

Last edited: Jul 4, 2005
3. Jul 4, 2005

### Yegor

Hm. This is what i got:
$$f(x)=\arctan(1/x); f'(x)=-\frac{1}{1+x^2}; f(x)\approx f(0)+f'(0)(x-0)=\pi/2-x$$
yes. it looks good

4. Jul 4, 2005

### Yegor

Great. i got next terms too. Thank you very much.