On 10 Maj, 19:13, Ray Vickson <RGVick...@shaw.ca> wrote: > On Thursday, May 9, 2013 11:01:54 AM UTC-7, JT wrote: > >http://www.wolframalpha.com/input/?i=0.499999999999999999999999999999... > > > n = -1. > > > 0.49999999999999999999999999999999999999999 = (n/2-1)/n > > >http://www.wolframalpha.com/input/?i=%3D%2810000000000000000000000000... > > > 0.49999999999999999999999999999999999999999=(100000000000000000000000000000000000000000/2-1)/ > > > 100000000000000000000000000000000000000000 > > > I do not understand to, can please someone explain why and how wolfram > > > get -1 for the upper calculation, it is obvious using the one below > > > what the solution is? > > > And if there was two solutions should not Wolfram give them both? What > > > is going on here, i am total newb to math calculators so tell me what > > > is going on? > > It must have something to do with truncation of floating-point numbers. I do not have access to Mathematica, but here is your example in Maple 14: > > eq:=z=(n/2-1)/n: lprint(eq); z = (1/2*n-1)/n <---equation > Nz:=solve(eq,n): lprint(Nz); -2/(2*z-1) <--- solution > > Default digits setting = 10 gives: > eq1:=0.49999999999999999999999999999999999999999 = (n/2-1)/n : > lprint(eq1); .49999999999999999999999999999999999999999 = (1/2*n-1)/n > solve(eq1,n); > 42 > 0.1000000000 10 > (this is 0.10e42) > subs(z=0.49999999999999999999999999999999999999999,Nz); > Float(-infinity) > > Even with the low digits setting, Maple handles the direct equation well, but fails when the parameter is > substituted into the solution (because the extra digits are handled in one problem but not in the other)' > > Now let's increase the digits setting: > > Digits:=60; > subs(z=0.49999999999999999999999999999999999999999,Nz); > 42 > 0.100000000000000000000000000000000000000000000000000000000000 10 > (this is 0.100....00 e42) > > Now both ways handle the extra digits well enough to yield identical answers. > > I presume something like this happens also in Mathematica. However, if by "Wolfram" you mean wolfram alpha, I don't know if it allows you to change digit settings easily.
Well honestly wolfram alpha gives answers both for mathematica and wolfram, so the cat is eating its tail here.