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Re: Why isn't Expand[] grouping terms by order?

• To: mathgroup at smc.vnet.net
• Subject: [mg87892] Re: Why isn't Expand[] grouping terms by order?
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Sat, 19 Apr 2008 03:37:03 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <fu9fq7$cbn$1@smc.vnet.net>

p at dirac.org wrote:

> Consider this:
> 
>    In[19]:= Expand[ (1+a*x)*(1+b*x), x]
> 
>                                  2
>    Out[19]= 1 + a x + b x + a b x
> 
> 
> How can I coax Mathematica to give me this, instead:
> 
>                                  2
>    Want[19]= 1 + (a + b) x + a b x
> 
> Why isn't Mathematica reporting the answer in terms of powers of x?

The function *Collect[]* is what you are looking for.

In[1]:= Collect[(1 + a*x)*(1 + b*x), x]

Out[1]= 1 + (a + b) x + a b x^2

In[2]:= Expand[(1 + a*x)*(1 + b*x), x]

Out[2]= 1 + a x + b x + a b x^2

In[3]:= Collect[%, x]

Out[3]= 1 + (a + b) x + a b x^2

Regards,
-- Jean-Marc