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Re: Help with Identities


You should use Equal (==) rather than SameQ (===)

Simplify[Log[x/y]==Log[x]-Log[y],#]& /@
  {{x>0, y>0}, {x>0, y<0}, {x<0, y>0}, {x<0, y<0}}

{True,False,True,True}

{Log[x/y], Log[x]-Log[y]} /. {x->2, y->-2}

{I*Pi, (-I)*Pi}


Bob Hanlon

> 
> From: "Sven C. Koehler" <schween at snafu.de>
To: mathgroup at smc.vnet.net
> Subject: [mg65591] [mg65585] Help with Identities
> 
> Hello!
> 
> As an occasional mathematican, I sometimes forget that i.e.
> 
> Log[x/y] is very similar to Log[x] / Log[y]
> 
> Is there some way in Mathematica to see how an mathematical
> expression could look like alternatively?  (Something like the  opposite
> of Simplify.)
> 
> And then I wonder why 
> 
> Log[x/y] === Log[x] - Log[y]
> 
> is False.  Can I instruct Mathematica to explain why this is False?
> 
> Best wishes,
> 
> Sven
> 
> 


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