How to do sums – not!

Ok, its maths time.  Now I can hear you complaining but we’ll just do some sums, plain addition, nothing fancy.

Get a pencil and paper.

No, you cant just pretend to get a pencil and paper, go get a pencil and paper.

Ok, now you have your tools ready, jot down the following numbers:

103130.41
1807.75
7390.11
9028.59
2831.26
1568.90
1794.71

Now, add them up.

Oh, ok, you can use a calculator, but just this once mind!

What did you get: 127551.73

Now, what happens if you add – 127551.73 (that’s minus 127551.73).  Yes, they cancel each other out and you get zero. Common law of addition – add a minus number and a plus equivalent and you get zero. Doesnt matter if you’re adding 4 to -4, or 4000000 to -4000000 it’s all the same zero.

Now, go open up ye ole MS Excel. Type in the numbers. First type in the list as above and use the sum function (as you do) to add them up. You should have something like this below-

Adding up

Now, in an empty cell type in -127551.73

Check that your sum function includes this new minus figure.

adding up more

Now, what’s that all about?

Did we make a mistake somewhere along the line, or is Excel programmed to add up in a not-so-conventional manner, or is this a lesson – computers can sometimes make a mess of things???

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Posted on May 11, 2008, in Uncategorized and tagged , , , . Bookmark the permalink. 3 Comments.

  1. Boredom Alert: http://www.dailydoseofexcel.com/archives/2007/06/06/subtracting-binary-fractions/

    Converting base 10 numbers (humans) to base 2 numbers (computers) and back causes little differences.

  2. pennybridged

    Indeed it does 🙂

  3. Huh? My head hurts!

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