Re: why is this happening?

*To*: mathgroup at smc.vnet.net*Subject*: [mg99225] Re: why is this happening?*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Thu, 30 Apr 2009 06:23:11 -0400 (EDT)*References*: <gt9aq0$q7o$1@smc.vnet.net>

Sean, After backsubstitution the answers Mathematica provides seem to be A- OK, so what's the problem? In[517]:= k1 ld + k1 k2 + ld^2 == 0 /. Solve[k1 ld + k1 k2 + ld^2 == 0, ld] // Simplify Out[517]= {True, True} In[516]:= k2/k1 + ld^2 + ld == 0 /. Solve[k2/k1 + ld^2 + ld == 0, ld] // Simplify Out[516]= {True, True} Cheers -- Sjoerd On Apr 29, 12:40 pm, sean_inc... at yahoo.com wrote: > Consider the following. > > Solve[k1 ld + k1 k2 + ld^2 == 0, ld] > > vs > > Solve[k2/k1 + ld^2 + ld == 0, ld] > > I expected to get the output I got from the first code. > > >From the second code, I was expecting something along the lines of > > ld ->-1 + (SquareRoot[1 - 4 (k2/k21)])/2 > ld -> -1 - (SquareRoot[1 - 4 (k2/k21)])/2 > > since that is the answer. > > Why is Mathematica turning the second scaled equation into the > unscaled one in the first before solving it? > > Thanks for any insight. > > Sean