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

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Emerald V

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

ab-ba.png

3 REPLIES 3
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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:

collab+-+12+12+13+summation+and+product+operator+iteration+01.jpg

Mathcad 11:

collab+-+12+12+13+summation+and+product+operator+iteration+02+M11.jpg

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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!

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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!

SUM.png

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