Re: Log Error
- To: mathgroup at smc.vnet.net
- Subject: [mg78832] Re: Log Error
- From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
- Date: Wed, 11 Jul 2007 05:59:43 -0400 (EDT)
- Organization: The Open University, Milton Keynes, UK
- References: <f6vnds$qfr$1@smc.vnet.net>
Laurence Kohn wrote:
> Hi,
>
> I had a question about something I found in Mathematica 5.2... I was
> plotting Log[x-1]-2 with x = {0,10} and y = {-10,5} and strangely found
> that the graph crossed the x-axes at approximately x == 8 ...
>
> It should be 101 of course, but it seemed like 8. Then I did an Nsolve
> with the equation set to 0, and low and behold got an answer x == 8.38906.
>
> Is this a bug in mathematica or am I missing something here?
No error from Mathematica here. You should post the exact code you
tried, because the solution to the equation Log[x - 1] - 2 == 0 is x ==
1 + E^2 (E being the base of the natural logarithms ~= 2.71828). You can
easily get the solution by hand or with the help of Mathematica.
Log[x - 1] - 2 == 0
Log[x - 1] == 2
E^Log[x - 1] == E^2
x - 1 == E^2
x == 1 + E^2
x ~= 8.39 (2 d.p.)
In[1]:= Plot[Log[x - 1] - 2, {x, 0, 10}, PlotRange -> {-10, 5}]
In[2]:= sol = ToRules[Reduce[Log[x - 1] - 2 == 0, {x}]]
Out[2]= {x -> 1 + E^2}
In[3]:= x /. sol
Out[3]= 1 + E^2
In[4]:= N[%]
Out[4]= 8.38905609893065
Regards,
Jean-Marc