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» Hatrack River Forum » Active Forums » Books, Films, Food and Culture » Calculator help (EDIT: Math help)

   
Author Topic: Calculator help (EDIT: Math help)
JumboWumbo
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I was trying to graph an equation when the message ERROR: INVALID DIM came up. How do I resolve this problem? It was working fine five minutes ago.

[ March 05, 2007, 12:33 AM: Message edited by: JumboWumbo ]

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RunningBear
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did the equation have an x(n) in it? If so, it needs to be written as x*(n).


what is the equation?

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rivka
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What brand and model of calculator?
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JumboWumbo
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TI-83 plus....but I fixed it. I found the (astronomically large) manual laying around and found out how to reset everything. Works fine now (which is unfortunate, because that means I have to keep on working).
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JumboWumbo
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Okay, now I need some math help. Here's what I'm working on:

1. The function,I(d)=Ae^(-kd) , gives the intensity of light, relative to the amount of light A at the surface for a given depth d in cm, where k is a constant equal to 0.0039cm^(-1). If we assume that A = 100%, I(d) will give the percent of light that remains at a depth of d cm.
a. Sketch a graph of this function. Is this a one to one function?
b. What percent of light is left at 200cm?

I'm still not getting a graph, but I don't think it's the calculator's fault. Any suggestions?

[ March 05, 2007, 12:47 AM: Message edited by: JumboWumbo ]

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Eisenoxyde
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Known: A=100%, A=1 k=0.0039^(-1)

so you need to type this into the calculator:

l(d)=e^(0.0039^(-1)*x)

What are your dimensions on your calculator?

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JumboWumbo
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Shouldn't it be -.0039? I've tried that, but still nothing.
Do you mean window dimensions? Because I've tried everything from xmin/xmax=-10/10 to -1000/1000. Same for ymin/ymax.

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just_me
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Did you figure it out yet?

I think the problem you're having is that .0039cm^1 is NOT -.0039 and it is NOT .0039^(-1)cm either. It IS .0039/cm.

So the equation you want to use in your TI-83 is
y1=e^(-.0039*X)

I suggest you try a window with x from 0 to 1000 and y from 0 to 1.1.

BTW - you probably WERE getting a graph with what you had in there it probably was just y=0 so you didn't see it.

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Happy Camper
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If you ever find yourself in another situation where you tried to graph something, and it works for a while, but nothing shows up, then what you could do is go into... Math (I think, it's been a while since I used an 83), and trace, the coordinates on the bottom will show you the X coordinate of the middle of the screen and the y coordinate of the function at that point, then you just fiddle with the bounds of your screen a bit till it graphs up all nice and shiny-like. This will really help you figure out if there's an error in your equation or just bad viewing boundaries. It does help if you have some idea what the function should look like.
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JumboWumbo
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JM- Yeah, that's what I ended up doing, though I had to ask a friend. I think I understand it now. Thanks for the advice though.

Happy- I'll be sure to (try and) remember that next time.

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Icarus
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The most common thing I see causing graphs to return errors like that is having stat plots turned on accidentally. Press the Y= button, and look at the top of the screen above Y1. You should see something like "Plot1 Plot2 Plot3" (I'm not sure because this is off the top of my head). If any of those is highlighted, and you didn't do it on purpose, press the up arrow until your cursor is over it, and press enter to turn it off.

This is unbelievably common and almost nobody knows how to fix it. Most people I know simply pull out the batteries and put them back in!

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