Subtraction-fest 11 ~ Checking Your Subtractions By Adding

If you have had to complete a subtraction problem and need to check your answer to ensure that you "got it right," there are a few methods that you could use.

You could, of course, repeat what you had already done! I.e. repeat all the same steps on paper or on your calculator ... but you risk repeating an error if that is all you do. It would be sensible to check your paper calculation using a calculator, or your calculator result by working out the answer on paper, but I want to share two alternate strategies.

The first strategy is simply performing the reverse operation. In mathematics we would call this the inverse operation/function. To check a subtraction, we simply ADD the two smaller numbers to obtain the larger one! This is handy and also relatively quick since most of us are rather better and more speedy with our adding skills!

In my next video I will demonstrate the second common checking method of "casting out nines."

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RATIONALE FOR THIS SERIES

Some of you have expressed quite some interest in different subtraction methods/algorithms. This resulted from my posting two lengthy videos in which I explained WHY different algorithms worked.

It became apparent, however, that I needed to create a series of much shorter videos in order to DEMONSTRATE these methods (and more) without the lengthy explanations in those first two videos.

[MrVoayer]

As you said, this is the basic test for correctness, thought to kids at school.

[raedunnit]

Well that was confusing. I learned math back in the 60's and the way they taught it them is the way I know how to do it. I have not issues with coming up with the right number....