FullSimplify and negative radicands
- To: mathgroup at smc.vnet.net
- Subject: [mg110199] FullSimplify and negative radicands
- From: Torsten Schoenfeld <kaffeetisch at gmx.de>
- Date: Mon, 7 Jun 2010 08:23:04 -0400 (EDT)
Why does FullSimplify think this equation is true even though x = -3 is clearly a counterexample? In[1]:= equation = Sqrt[x^2 - 1] == Sqrt[x + 1] Sqrt[x - 1] Out[1]= Sqrt[-1 + x^2] == Sqrt[-1 + x] Sqrt[1 + x] In[2]:= $Assumptions Out[2]= True In[3]:= equation // FullSimplify Out[3]= True In[4]:= equation /. x -> -3 Out[4]= False (This is with Mathematica 7.0.1.0.)