Introducing double negatives.

Yesterday’s math lesson with Dearie turned out to be rather interesting.  We were going over a review page and the question was to find the side of a square if given the perimeter.  She made the mistake of thinking that the perimeter was the area and came up with the wrong answer.  It was not an understanding mistake (she just mixed up the perimeter and area), so I just gave her a few more to help her remember the difference.  She realized that I was making them up and called me on it.  I said yes, and that I was sorry that I couldn’t use her favorite number (-18) in any of the problems.  After all, the side of a square cannot be -18.  But then, being the math geek that I am, I did give her some problems with her favorite number. 

20+(-18)=  She thought a while and said 2.   Correct

-18 – (-18)= She was puzzled by this one.  She said -36, and I replied, “Nope.”  She thought a while longer and with a smile on her face said 0.  Correct

I was really surprised that I did not have to explain this concept to her.  Double negatives confuse a lot of people, and to figure it out on her own was pretty amazing.  No wonder she is bored with the multiplication and division of decimals that her math book is covering right now.  I am trying to compact it for her, but it is still not fast enough to get to the good stuff.


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