Re: RE: Limit involving square root
- To: mathgroup at smc.vnet.net
- Subject: [mg30246] Re: [mg30204] RE: [mg30167] Limit involving square root
- From: BobHanlon at aol.com
- Date: Fri, 3 Aug 2001 00:56:16 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
In a message dated 2001/8/2 5:14:39 AM, BradleyS at Harker.org writes:
>Your eyes (and domain) are deceiving you. At first glance, I thought the
>result to the first function should've been zero. Then after evaluating,
>at
>what I thought were very large numbers, I thought I was mistaken and indeed
>the answer was 5. But further investigations reveal that the first limit
>is
>in fact 0. Try this to see something interesting. Evaluate at 10^15,
>10^16
>and 10^17. I think you will be surprised. Also, I tried using Mathematica
>4.1 to evaluate the first limit directly and it just gave me back my input.
>
The limit is 5. You are getting numerical errors due to machine precision.
Use extended precision.
N[(x - Sqrt[9 - 10 x + x^2]) /. { {x -> 1*^15}, {x -> 1*^16}, {x ->
1*^17}, {x -> 1*^18}}, 50]
The limit is easier to understand if you write the expression as
x-Sqrt[(x-5)^2-16]
Bob Hanlon
Chantilly, VA USA