Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: Re: [mg5119] Integration..Constants
Replies: 0

 Carl K. Woll Posts: 257 Registered: 12/7/04
Re: [mg5119] Integration..Constants
Posted: Nov 7, 1996 12:44 AM

On Wed, 30 Oct 1996, Dennis Michael Winters wrote:

> I have tried several times to perform an integration(math..2.2.3)B
> similar to the following.
>
> In[1]:=Integrate[(ao+x)^-1 (bo^-2), x]
>
> The results, however, are always incorrect:
>
> Out[1]=-ln(-ao+x) <---where the constant "ao" should be positive.
>
> ... more stuff

Dennis,

It seems to me that you made a typo in your question. Specifically, if you
give Mma the integral

In[1]:=Integrate[(ao-x)^-1, x]

then you probably get (I haven't checked this on Mma myself)

Out[1]:=-ln[-ao+x]

But, try taking a derivative of this, you get the expected answer. You
might have expected to get

Out[1]:= -ln[ao-x]

and, naturally enough, if you take a derivative of this, you get the
expected answer. So, what's going on? Well, these two answers differ by a
constant:

-ln[-ao+x] = -ln[ (-1) (ao-x) ] = -ln[-1] - ln[ao-x]

and

-ln[-1] = -ln[ Exp[I Pi] ] = - I Pi

Thus, Mma is not doing anything wrong here.

Carl
Physics Dept
U of Washington

carlw@u.washington.edu