ZeroSum Ruler (home)

Blogging on math education and other related things

Subtraction as “as compared to” December 10, 2010

I learned the best thing today at an interview, which I know sounds a bit weird.  Usually at interviews it’s all about what I’ve done and where I see myself in 5 years and whatnot.  This interview was far different, and better, and awesome.  Today at my interview, I learned something really great about… subtraction.

 

Let’s take a problem like “7 - 2“.  We can read the subtraction sign as “as compared to“.  When we find 7 and we find 2 on a number line, comparably they are 5 apart.  And indeed, 7 - 2 = 5.

But does it work for subtracting negatives?  let’s check…

 

“7 - -2″.  Seven as compared to negative 2.    7 and -2 are 9 apart, and in fact    7 -  -2   =   7 + 2 =   9!

 

This is a SUPER model that, as my boyfriend just said, opens a whole new world.  Who knew at 33 I’d still be learning subtraction! 

 

Awesome!

 

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