Thread Subject:

int in R2008b; wrong result?

Just a simple integral in R2008b:

>> int ('exp(x)/(exp(x) + 42)')
 
ans =
 
(exp(x)*log(exp(x) - 41))/(42*(exp(x) - 42))

Using school mathematics (substitution z=exp(x)+42) I get:

log (exp (x) + 42)

I can't see that both results are identical.

"Joerg Buchholz" <buchholz@hs-bremen.de> wrote in message <ghrhfd$ib6$1@fred.mathworks.com>...
> Just a simple integral in R2008b:
>
> >> int ('exp(x)/(exp(x) + 42)')
>
> ans =
>
> (exp(x)*log(exp(x) - 41))/(42*(exp(x) - 42))
>
> Using school mathematics (substitution z=exp(x)+42) I get:
>
> log (exp (x) + 42)
>
> I can't see that both results are identical.

It looks like there is something wrong with int here. The result returned from matlab R2008b is not equivalent to the correct answer from using high school calculus. R2008a gives the correct answer. I hope they release a patch to fix this.

"Tazmusica " <tazmusica2@deletethis.gmail.com> wrote in message <gi93do$9tq$1@fred.mathworks.com>...
> "Joerg Buchholz" <buchholz@hs-bremen.de> wrote in message <ghrhfd$ib6$1@fred.mathworks.com>...
> > Just a simple integral in R2008b:
> >
> > >> int ('exp(x)/(exp(x) + 42)')
> >
> > ans =
> >
> > (exp(x)*log(exp(x) - 41))/(42*(exp(x) - 42))
> >
> > Using school mathematics (substitution z=exp(x)+42) I get:
> >
> > log (exp (x) + 42)
> >
> > I can't see that both results are identical.
>
> It looks like there is something wrong with int here. The result returned from matlab R2008b is not equivalent to the correct answer from using high school calculus. R2008a gives the correct answer. I hope they release a patch to fix this.

If this is indeed the case, you should file a bug report directly with MathWorks.

someone wrote:
> "Tazmusica " <tazmusica2@deletethis.gmail.com> wrote in message <gi93do$9tq$1@fred.mathworks.com>...
>> "Joerg Buchholz" <buchholz@hs-bremen.de> wrote in message <ghrhfd$ib6$1@fred.mathworks.com>...
>>> Just a simple integral in R2008b:
>>>
>>>>> int ('exp(x)/(exp(x) + 42)')
>>>
>>> ans =
>>>
>>> (exp(x)*log(exp(x) - 41))/(42*(exp(x) - 42))
>>>
>>> Using school mathematics (substitution z=exp(x)+42) I get:
>>>
>>> log (exp (x) + 42)
>>>
>>> I can't see that both results are identical.
>> It looks like there is something wrong with int here. The result returned from matlab R2008b is not equivalent to the correct answer from using high school calculus. R2008a gives the correct answer. I hope they release a patch to fix this.
>
> If this is indeed the case, you should file a bug report directly with MathWorks.
>

This bug has been reported, and will be fixed in a future release. Sorry
about that, folks.

Alan Weiss
MATLAB mathematical toolbox documentation

The bug has not been fixed in R2009a.