Get Help

Turn on suggestions

Auto-suggest helps you quickly narrow down your search results by suggesting possible matches as you type.

Showing results for

- Community
- :
- PTC Mathcad
- :
- PTC Mathcad
- :
- Ward No. 21: a+b is not equal b+a

Options

- Subscribe to RSS Feed
- Mark Topic as New
- Mark Topic as Read
- Float this Topic for Current User
- Bookmark
- Subscribe
- Mute
- Printer Friendly Page

Auto-suggest helps you quickly narrow down your search results by suggesting possible matches as you type.

Showing results for

Highlighted
##

- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Notify Moderator

12-04-2012
12:03 PM

12-04-2012
12:03 PM

Ward No. 21: a+b is not equal b+a

3 REPLIES 3

Highlighted
##

- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Notify Moderator

12-13-2012
04:54 AM

12-13-2012
04:54 AM

Re: Ward No. 21: a+b is not equal b+a

Interesting. The 'feature' has been there since at least Mathcad 11 (although giving a slightly different result than M15). Similar thing happens with the product operator.

Stuart

Mathcad 15:

Mathcad 11:

Highlighted
##

- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Notify Moderator

12-13-2012
06:28 AM

12-13-2012
06:28 AM

Re: Ward No. 21: a+b is not equal b+a

Yes! a*b is not equal b*a

I hope I understand this error.

Mathcad's symbolic engine solves a quadratic equation and chooses not correct root.

And second.

I have seen in one Mathcad guide that in sum(f(i), i=a..b) b must be more than a always!

Highlighted
##

- Mark as New
- Bookmark
- Subscribe
- Mute
- Subscribe to RSS Feed
- Permalink
- Email to a Friend
- Notify Moderator

12-17-2012
08:49 AM

12-17-2012
08:49 AM

Re: Ward No. 21: a+b is not equal b+a

Computer math is not commutative as we operate on a finite set of numbers in contrary to the infinite real numbers and because of rounding errors done by decimal-binary-decimal conversion (which could be avoided by using BDC arithmetic).

The second example using the symbolic evaluation is quite different. Don' think that it's a bug. Rather Mathcad is not going to give you all possible solutions. If n is undefined Mathcad does not know wheter its going to sum UP or a sum DOWN, so its defaults to UP and gives you that solution. Only the last example below seems to be a bug, but the assume modifier never was reliable anyway.

WE

Correction: Even the last exemple has its logic. As the default seems to be that the upper limit is greater than or equal to the lower limit, the modifier n>=1 can only mean that n=1. Tricky!