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I found exponential regression on Desmos for a few values:

enter image description here

I want to find the X value when Y is 40,000. So, when zooming in on 40,000, I see that X is around 160. But, when putting the equation:

y=7.4927(1.0695)^x

in Wolfram Alpha, I get x = 127.735. Why are the two answers different?

Thanks so much,

Ruchir

Gerry Myerson
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Ruchir Baronia
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1 Answers1

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You are looking at the $y$ value 400,000, not 40,000!

If you solve $7.4927 (1.0695)^x = 400,000$ numerically, you should get $127.735$, just like the answer from Wolfram Alpha.

enter image description here

GaussTheBauss
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  • I came to the same conclusion: the 400k vs 40 issue. Looks like an extra zero typo somewhere by the OP. – rschwieb Dec 28 '15 at 14:57
  • Oh my gosh. Is there anyway to actually pinpoint the dot at Y = 40,000 and see the X value there? I can never see that X value properly, as when I place the dot on the line Y=40,000 it shows me coordinates for Y=40,001. Please explain how to get the coordinates. I will accept your answer once I confirm by pinpointing the 40,000 on the graph, but I'm unable to do that. I have liked your post 400,000 times! :) – Ruchir Baronia Dec 28 '15 at 14:58
  • @RuchirBaronia Unfortunately, I don't think Desmos supports solving for $x$ given $y$. But there is a way around this. You could graph $y=40,000$. and find the intersection point between this line and your regression line! I hope this helps! – GaussTheBauss Dec 28 '15 at 15:12
  • @GaussTheBauss Hey, that helps a lot. But, how do I graph y = 40,000? Feel free to post an answer! Thanks – Ruchir Baronia Dec 28 '15 at 15:14
  • @RuchirBaronia just type $y=40000$ on the right. – GaussTheBauss Dec 28 '15 at 15:16
  • @RuchirBaronia I have added a picture. It helps to change the range of $x$ and $y$ to find the intersection point. – GaussTheBauss Dec 28 '15 at 15:21