# Math Tricks, Negative Space, and Simple Beauty

Once again we start with two of my favorite things: soccer and math. I’ve talked about them both at length, for example in my “geometry in soccer” post from March 2009. Both are related by a similar underlying, structured framework. Both have rules, methods, and strategies for finding success, whether that’s solving a problem or winning a game.

What most non-players don’t understand is that despite the rules that govern both math and soccer, there are tricks to the game as well. These are the visions and insights that exist not within the simple rules and methods of an operation or a play, but rather in the negative space – the non-obvious space surrounding the operations and plays. You may find, more often than not, that recognizing these tricks in all aspects of life can provide the competing advantage necessary for happiness and success.

The soccer tricks will have to wait until after some knee surgery, so for now, I’ll stick with the math. There are thousands of known tricks in math, and probably an infinitesimal amount of unknown tricks waiting for an epiphany of recognition. Here’s an example:

Squaring Any Number Ending in “5”

Although this works for any number that ends in 5, it’s probably most practical for two digit numbers when no calculator is present. Let’s use 65 as an example, where we try to quickly compute 65 squared, or 65^2.

All you have to do is look at the number to the left of the “5” in the ones place. For our example, we have a “6”. Multiply this number by the number that follows it sequentially, which is “7” for our example. We get 6*7=42. To find our final answer of 65^2, all we have to do is take the result of our multiplication and append a “25” to the end of it, recognizing that the last two digits of the square of any number ending in “5” will always be “25”.

So for our example, we have “42” + “25” which gives us 4,225. The square of 65 is 4,225. Pretty neat, huh? Try it with some others…

25^2:     2*3=6,             “6” + “25”        = 625
95^2:     9*10=90,         “90” + “25”     = 9,025
475^2:   47*48=2,256, “2256” + “25” = 225,625

For a proof, I’ve looked to Dr. Math at the MathForum.org website. Here goes:

Let’s generalize a two-digit number ending in “5” by the representation X5, where X could be 1, 2, .., 8, or 9. Essentially, X5 is really a shorthand notation for the integer represented by

10*X + 5

Let’s go ahead and square X5:

(X5)^2 = (10*X + 5)^2 = (10*X + 5)*(10*X + 5) = 100*X^2 + 100*X + 25

Now factor our the 100 and an X from the first two terms:

= 100(X^2 + X) + 25 = 100*X*(X+1) + 25

Looking at this closely, you can see that this is exactly the product of X and the next sequential integer (X+1) with “25” appended to the end. Pretty cool, huh?

Notice that this trick works for squaring any integer that ends in “5”, not just two-digit numbers. Dr. Math shows us that for the the larger proof would have to be modified a bit (since all integers that end in “5” cannot be represented by 10*X + 5).

Seemingly Complex, But Beautifully Simple

Although the rules and structure of math may at times seem complex and chaotic, in the negative space of math we can find a beautiful simplicity through which things can fall in place. The same can be true for soccer, language, love, astronomy, cooking, and all aspects of life. Sometimes we’ve defined a framework (or have had it defined for us) of rules and methods to follow. But if we take a step back, look between the numbers and think outside the box, maybe we’ll find a simpler route to happiness and success.