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