Really strange problem, for very bright people

Hi all,
 
I'm looking for some really bright people to help with this one, because in
my entire life, I've never seen something as strange as this.
 
We have a calculation engine that does a whole whack of calculations...on we
picked up a bug with it. We then stripped down the calculation to bare bones
and were quite amazed at what we found....below is a code snippet to explain
what's happening...
 
var
  vCalc : extended;
 
vCalc := (4.328247 * ( 33660 / 1000 ) * 1.06 * 1 * 1 * 1.5 * 1.035 ) - ( 1.5
* 1 * ( 33660 / 1000 ) * 4.328247 * 1.06 * 1.035 );
 
This gives an answer of 2.7755575616e-17
 
Now do this....
 
vCalc := (4.328247 * ( 33660 / 1000 ) * 1.06 * 1 * 1 * 1.5 * 1.035 ) - (
4.328247 * 1.06 * 1.5 * 1 * ( 33660 / 1000 ) * 1.035 );
 
You'll notice that I just moved the 4.328247 and 1.06 to the front of the of
the second brackets and you get an answer of 1.3877787808e-17
 
Now try this...
 
vCalc := (4.328247 * ( 33660 / 1000 ) * 1.06 * 1 * 1 * 1.5 * 1.035 ) - (
4.328247 * 1.06 * 1.035 * 1.5 * 1 * ( 33660 / 1000 ) );
 
All I've done now is move the 1.035 after the 1.06 in the second brackets
and I get an answer of 0 (which is actually the correct answer, which we
expected, and which Excel gives us as well)
 
Just moving the value around in a multiplication calculation produces
different results....the different results are consistent with single,
double and extended types, but with comp and currency types, the calculation
is correct every time....
 
Can anyone explain this?
 
Thanks
Steve
 


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