Obnoxious Posted March 30, 2005 Share Posted March 30, 2005 Observe: 2 = 2 2² = 2² 4 = 4 Unsquare both sides, and you get either 2 = 2 or -2 = 2 And if you square both sides again, you get 4 = 4 again So, does -2 = 2? Or am I just an idiot? Link to comment Share on other sites More sharing options...
ecoli Posted March 30, 2005 Share Posted March 30, 2005 taking the square root of both sides would yield 2=2 and -2=-2 Link to comment Share on other sites More sharing options...
Obnoxious Posted March 30, 2005 Author Share Posted March 30, 2005 but -2 = 2 also comes out as a possibility. Link to comment Share on other sites More sharing options...
calbiterol Posted March 30, 2005 Share Posted March 30, 2005 sorry, it doesn't work that way - both sides are going to be +/- 2... that's just the way it is... Link to comment Share on other sites More sharing options...
Callipygous Posted March 30, 2005 Share Posted March 30, 2005 you just broke our presentation of math. the fact is -2 does not equal 2 (obviously). just because the answer could be -2 and 2 does not mean that it actually works. when solving this in a real life equation if you left it at that, instead of plugging things in (or using logic) to remove one of the answers you would get a big fat "0". try telling your teacher its -2 feet some time. Link to comment Share on other sites More sharing options...
ydoaPs Posted March 30, 2005 Share Posted March 30, 2005 you can if you are taling about trig:) Link to comment Share on other sites More sharing options...
Dak Posted March 30, 2005 Share Posted March 30, 2005 the equasion reads "plus or minus 2" = "plus or minus two", which is correct as both sides of the equasion are equal to keep it an equity, you have to do the same things to both sides, right? so you cant just take the modulouse of one side and the modulouse*-1 of the otherside to get 2 = -2. so for example, 4 = 4, [math] \sqrt{4} = \sqrt{4}, [/math] +or- 2 = +or- 2, 4 = 4, heres a better one, as far as proving the incorrect goes: a = b (times by a gives) [math]a^2 = ab [/math] (minus [math]b^2[/math] gives) [math]a^2 - b^2 = ab - b^2[/math] (bracketise (forget the correct term, sorry)) (a + b)(a - b) = b(a - b) (now divide by (a - b)) a + b = b (now, as a = b, substitute b for a to get) b + b = b (ie) 2b = b (divide by b) 2 = 1 tada! its not correct, but it seems like i, too, have broken math. Link to comment Share on other sites More sharing options...
ydoaPs Posted March 30, 2005 Share Posted March 30, 2005 another unknown error. what is going on? Link to comment Share on other sites More sharing options...
Dak Posted March 30, 2005 Share Posted March 30, 2005 my fault: 1st time using [math] \LaTeX [/math] hang on, ill fix my last post :::EDIT::: hmm, well athetics aside atleast its readable now Link to comment Share on other sites More sharing options...
ed84c Posted March 30, 2005 Share Posted March 30, 2005 Observe:2 = 2 2² = 2² 4 = 4 Unsquare both sides' date=' and you get either 2 = 2 or -2 = 2 And if you square both sides again, you get 4 = 4 again So, does -2 = 2? Or am I just an idiot?[/quote'] wack off a google image search for quadratic parabola Link to comment Share on other sites More sharing options...
Klaynos Posted March 30, 2005 Share Posted March 30, 2005 When the +/- sign is used on both sides of an equation (eg in double angle trig identities) it is taken that if + is chosen on one side + is chosen on the other side, BUT it is possible (also in double angle trig identities there is an exaple of this) is to have a -/+ sign on one side and +/- on the other in which case you pick + on one side and - on the other. So in the case of a squar rout it is always written as +/- so one is picked and used on both sides... Link to comment Share on other sites More sharing options...
mmalluck Posted March 30, 2005 Share Posted March 30, 2005 Dak: You divide by (a-b), but from the very begining a=b, so when you are dividing (a-b) you are dividing by zero. That's a big math no-no. Ta da! Link to comment Share on other sites More sharing options...
Dak Posted March 30, 2005 Share Posted March 30, 2005 yup! nice1. took me absolutely ages to spot that one when i first saw it there are a couple more on this site, inclluding another x = -x one involving square-roots like the OP Link to comment Share on other sites More sharing options...
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