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Re: simplify a trig expression

• To: mathgroup at smc.vnet.net
• Subject: [mg65456] Re: simplify a trig expression
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Mon, 3 Apr 2006 06:59:21 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <200603311109.GAA15029@smc.vnet.net> <200604011038.FAA07301@smc.vnet.net> <e0o58m$1vf$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Murray Eisenberg wrote:
> OK, now how about the following--another case where Mathematica gives a 
> more complicated looking answer than the typical paper-and-pencil direct 
> substitution would provide?
> 
> Mathematica gives:
> 
>    Integrate[Sin[x]/(1 - Cos[x]), x]
> 2*Log[Sin[x/2]]
> 
> I'd like an answer in the form of Log[1-Cos[x]]  (plus a constant, to 
> actually equal the above).  The best I've been able to do so far is:
> 
>    (Integrate[Sin[x]/(1 - Cos[x]), x]
>       // Simplify[# /. a_*Log[b_]:>Log[b^a]] &
>       //MapAt[TrigReduce, #, 1]&) /. (Log[c_ b_]->Log[b]+Log[c])
> -Log[2] + Log[1 - Cos[x]]
> 
> Is there some easier way?
> 
> Andrzej Kozlowski wrote:
>> This is one of those cases where FullSimplify will not work because  
>> it lacks a suitable transformation function. In this particular case  
>> the transformation function is of the form:
>>
>> f[n_*Log[a_]] := Log[a^n]
>>
>> Of course this is only valid with various assumptions on n and a, but  
>> I won't bother with this here. Anyway, observe that:
>>
>>
>> FullSimplify[Integrate[Cos[x]/(Sin[x] + 1), x],
>>    TransformationFunctions -> {Automatic, f}]
>>
>>
>> Log[Sin[x] + 1]
>>
>> Note also that Simplify will not work even when you add f.
>>
>> I am not sure if there are good reasons for adding a version of f  
>> (taking account of suitable assumptions) to the default  
>> transformation functions of FullSimplify. It may however be a good  
>> idea to have another possible value for the option  
>> TransformationFunctions besides only Automatic and user defined ones.  
>> In fact I have suggested in the past one or two other useful  
>> TransformationFunctions; perhaps it might be a good idea to define  
>> more and  collect them into a single option value or maybe several.
>>
>> Andrzej Kozlowski
>>
>>
>>
>>
>>
>>
>> On 31 Mar 2006, at 13:09, Murray Eisenberg wrote:
>>
>>> A direct substitution (with paper and pencil) gives that the  
>>> integral of
>>>   Cos[x]/(Sin[x] + 1) is Log[Sin[x] + 1].  This is valid provided  
>>> Sin[x]
>>> is not -1.
>>>
>>> Mathematica gives:
>>>
>>>    Integrate[Cos[x]/(Sin[x] + 1), x]
>>> 2 Log[Cos[x/2] + Sin[x/2]]
>>>
>>> Is there some simple way to coerce the latter Mathematica-supplied
>>> result into the paper-and-pencil answer?
>>>
>>> The closest I could get is:
>>>
>>>    Log[TrigExpand[Expand[(Cos[x/2] + Sin[x/2])^2]]] /.
>>>    {Sin[x/2] -> Sqrt[(1 - Cos[x])/2],
>>>     Cos[x/2] -> Sqrt[(1 + Cos[x])/2]}
>>> Log[1 + Sqrt[1 - Cos[x]]*Sqrt[1 + Cos[x]]]
>>>
>>> Am I not seeing some easier TrigExpand or TrigReduce method?

Hi Murray,

What about the following?

In[1]:=
Integrate[Sin[x]/(1 - Cos[x]), x] /. (n_)*Log[x_] :>
    PowerExpand[Log[TrigReduce[x^n]]]

Out[1]=
-Log[2] + Log[1 - Cos[x]]

In[2]:=
Integrate[Cos[x]/(Sin[x] + 1), x] /. (n_)*Log[x_] :>
    PowerExpand[Log[TrigReduce[x^n]]]

Out[2]=
Log[1 + Sin[x]]

Best regards,
JM

• References:
  • Re: simplify a trig expression
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>