Which theorem does every schoolboy know? The Pythagorean Theorem, of course!

In a right triangle, in a plane, the sum of the squares of the legs equals the square of the hypotenuse. (The Scarecrow gets it wrong. He should have held out for a real brain. Or a slide rule.)

That’s $a^2 + b^2 = c^2$ for those who like their mathematical laws brief and symbolic.

So Friday I taught the theorem to my algebra classes. And tomorrow we will talk about today’s event: May 12 2013, 5/12/13, a calendar date that makes a Pythagorean triple $5^2 + 12^2 = 13^2$

Of course we will waste spend considerable class time tomorrow moaning the fact that so few of us marked this special event, but cheerfully noticing that we get another chance in such a short time. And then we will engage in the tedious business of locating when else in our lifetimes we have allowed such a date to pass, without noticing, and when else in our lifetimes such a date will arrive, for us to celebrate.

Others may celebrate π Day, or 2π day, or e day (February 7, 2018, by my reckoning, a big day here if this blog still exists). A few years back I dismissed “square root day. But Pythagoras Days are neither so common as to be ho hum, nor so rare as to be once in a lifetime. I have long appreciated them.

Pythagoras day is clearly the prince of the mathematical holidays. I hope you enjoyed yours.

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