Re: Self-teaching snag

• To: mathgroup at smc.vnet.net
• Subject: [mg74578] Re: [mg74556] Self-teaching snag
• From: "Chris Chiasson" <chris at chiasson.name>
• Date: Tue, 27 Mar 2007 03:58:11 -0500 (EST)
• References: <200703260704.CAA11373@smc.vnet.net>

```I forgot to answer the question: about 37 days

In[1]:= RSolve[charge[i+1]==95/100*charge[i]&&charge[0]==1,charge[i],i]
charge[i]==.15/.%[[1]]
Solve[%,i]

Out[1]= {{charge[i]->(19/20)^i}}

Out[2]= (19/20)^i==0.15

Solve::ifun: Inverse functions are being used by Solve, so some
solutions may not be found; use Reduce for complete solution
information.

Out[3]= {{i->36.9857}}

On 3/26/07, Todd Allen <genesplicer28 at yahoo.com> wrote:
> Hi All,
>
>    I am trying to refresh my skills in basic problem
> solving using Mathematica, but am running into some
> difficulties which are beginning to make me suspicious
> of Mathematica itself.  (I probably should be
> suspicious of my own brain...but you know how that is
> :-)
>
> Here is the scenario:  I have written a basic function
> to tell me what percentage of battery power will
> remain in a battery after x number of days, provided
> that we start with a full charge and lose 5% of that
> charge per day.
>
> If you execute the following code in Mathematica
> (V5.1):
>
> charge[0]=1.0 (* 100% *);
> charge[day_]:=(charge[day-1]-(0.05*charge[day-1]));
> charge[20]
>
> I receive an output of 0.358486 for my query at the 20
> day mark.....so, no problem so far.
>
> However, when I try to ask for the output at
> charge[35], mathematica seems to enter an endless
> calculation.  I've let the computer run for as long as
> 5 minutes without getting an answer.  Is there
> something wrong with my function, my version of
> Mathematica or something else I haven't considered?
>
>
>
> When I try the following:
>
> In[145]:=
> Solve[charge[day]==0.15,day];
>
> Mathematica gives me the error:
> "\$RecursionLimit::reclim: Recursion depth of 256
> exceeded."
>
> I am trying to ask Mathematica to tell my how many
> days it takes to reduce the battery power to 15
> percent, but I must be messing something up??
>
> If anyone has any pointers, I'd certainly appreciate
> it, because I am a little stuck right now.
>
> Best regards,
> Todd Allen
>
>
>
> ____________________________________________________________________________________
> We won't tell. Get more on shows you hate to love
> (and love to hate): Yahoo! TV's Guilty Pleasures list.
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>
>

--
http://chris.chiasson.name/

```

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