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I apologize if I cannot help anybody with my question right now. It's simply that I don't know what type of function represents the data I have plotted in excel, and therefore I cannot specify so in my question.

The problem is that I have a set of data as follows:

X      Y
0   262.44
10  246.93
20  44.82
30  5.05
40  4.51
50  4.39
60  3.28
70  2.75

The graph I get in excel looks as follows, but I simply cannot identify the function that produces this line. enter image description here

It is not exponential or logarithmic, and none of Excels build in line functions will fit it. Can anyone help me in identifying it's equation?

Once again, I'm really sorry if it doesn't contribute to others right now, but it'm desperately pressed for time and I could not think of a more informative title

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Micrified
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  • It appears to be a regressive sin function, but I really am drawing blanks.... – Micrified Mar 13 '14 at 21:41
  • There are a number of functions that fit that general shape; I'm not sure I would rely on any particular fit, though, without several more data points between 10 and 20 (or between 0 and 30) to better reveal the function's shape. can you say anything about where your data came from that might give clues as to what one might expect the shape to actually be? Given that the $x$-axis is an angle measure, I might try plotting intensity against $\sin \theta$ rather than against $\theta$ to see if the shape looks more obvious. (and also with the other trig functions) –  Mar 13 '14 at 21:44
  • Maybe $-\arctan(x)$? – Uri Brezner Mar 13 '14 at 21:48
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    @Hurkyl This data represents light intensity readings from a sensor as it is incrementally angles away from a light source. The X axis represents the angular displacement, and the Y axis the corresponding light intensity (lux). – Micrified Mar 13 '14 at 21:48
  • @Amzoti It appears that the quartic is the best fit. I suppose I should go with that? Thanks so much for this link. I was trying to figure out how to use WolfRam earlier but didn't know how to tell it to distinguish between X and Y values. – Micrified Mar 13 '14 at 22:08
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    @Owatch: Based on Malus's law, I would expect trying to fit a plot of $I$ versus $\cos^2 \theta$ would be easier than trying to fit a plot of $I$ versus $\theta$. –  Mar 13 '14 at 22:13

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