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MathGroup Archive 2007

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Re: Self-teaching snag

  • To: mathgroup at smc.vnet.net
  • Subject: [mg74592] Re: Self-teaching snag
  • From: siewsk at bp.com
  • Date: Tue, 27 Mar 2007 04:05:46 -0500 (EST)
  • References: <eu7re0$b6p$1@smc.vnet.net>

On Mar 26, 5:06 pm, Todd Allen <genesplice... 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?
>
> Additionally,
>
> 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.http://tv.yahoo.com/collections/265

What you have is basically an exponential decay function. So you can
use the exponential decay function directly

Solve[Exp[x]==0.95,x]
x is -0.0512933

decay[day_]:=Exp[ -0.0512933 * day ]

Compare

charge[0]
decay[0]

charge[1]
decay[1]

charge[2]
decay[2]

Now we can solve for the day when it reaches 0.15

Solve[decay[day]==0.15,day]
day is 36.9857




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