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I was seeing some equations in WA, and i got with http://www.wolframalpha.com/input/?i=%28k%2B1%29%5E2%3E%3D4%28k-1%29%5E2

Let's manually solve the equation $$(k+1)^2\ge4(k-1)^2$$ $$(k+1)^2-(2k-2)^2\ge0$$ $$(-k+3)(3k-1)\ge0$$ But in alternate forms, it gives $$(k-3)\left(k-\frac13\right)\ge0$$ What is happening here?

chubakueno
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  • Shouldn't this be on: http://mathematica.stackexchange.com/ – Ryan Budney Aug 11 '13 at 23:57
  • It's a bug. Somebody should report it. – Daniel Fischer Aug 11 '13 at 23:57
  • @DanielFischer How do you do that? – chubakueno Aug 12 '13 at 00:01
  • I have no idea. Maybe there's something on the "about" page or so, maybe one can only report a bug with an account, but perhaps someone on Mathematica.se knows. – Daniel Fischer Aug 12 '13 at 00:04
  • @DanielFischer I sent an email to their "support" mail. I guess that is the best I can do for now. – chubakueno Aug 12 '13 at 00:12
  • That should do, methinks. Well done, and good find. – Daniel Fischer Aug 12 '13 at 00:13
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    WolframAlpha bugs can be reported via the "Give us your feedback" window at the bottom of the page. This is nice since the input and result you received are automatically included. – Mark McClure Aug 12 '13 at 01:57
  • @MarkMcClure thank you, now I also reported it from there to have more chance of success. – chubakueno Aug 12 '13 at 02:01
  • Your last step is wrong. If you multiply (-k+3)(3k-1)>=0 with -1/3 then sign of inequality will change. It will be (k-3)(k-1/3)<=0 instead of ">=0". –  Aug 02 '14 at 19:25
  • @Mathematician171 There is no step between the third and fourth line, the fourth was(the error has been fixed) WA's bogus reult :) – chubakueno Aug 02 '14 at 22:10
  • I checked the site just now and the inequality is in the correct direction (i.e. reversed from the original). To be more explicit, the site is now listing $(k-3)(k - \frac 13) \leq 0$ as one of the alternate forms, which is correct. Has the bug been fixed? I assume there was no typo in the original question? – Deepak Jun 03 '15 at 12:31
  • @Deepak Yes, it has been fixed :) – chubakueno Jun 04 '15 at 16:17

1 Answers1

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It appears to be an error. I have reported it, and received this message back from WolframAlpha:

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A.S
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