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Get monthly math tips and activity ideas, and be the first to hear about new books, revisions, and sales or other promotions. Sign up for my Tabletop Academy Press Updates email list.

I just discovered this fun Pi Day song from The Singing Nerd. Definitely need to add him to my YouTube subscriptions.

Hat tip: Singing Banana.

Get monthly math tips and activity ideas, and be the first to hear about new books, revisions, and sales or other promotions. Sign up for my Tabletop Academy Press Updates email list.

*[Feature photo above by Olga Lednichenko via Flickr (CC BY 2.0).]*

This week I have a series of quotes about calculus from my first two years of blogging. The posts were so short that I won’t bother to link you back to them, but math humor keeps well over the years, and W. W. Sawyer is (as always) insightful.

**I hope you enjoy this “Throw-back Thursday” blast from the Let’s Play Math! blog archives:**

Eldest daughter had her first calculus lesson last night: finding the limit as delta-*t* approached zero. The teacher found the speed of a car at a given point by using the distance function, calculating the average speed over shorter and shorter time intervals. Dd summarized the lesson for me:

“If you want to divide by zero, you have to sneak up on it from behind.”

This kicked off my week with a laugh:

Today I said to the calculus students, “I know, you’re looking at this series and you don’t see what I’m warning you about. You look and it and you think, ‘I trust this series. I would take candy from this series. I would get in a car with this series.’ But I’m going to warn you, this series is out to get you. Always remember: The harmonic series diverges. Never forget it.”

—Rudbeckia Hirta

Learning Curves Blog: The Harmonic Series

quoting Alexandre Borovik

Rudbeckia Hirta has a great idea for a new TV blockbuster:

And here’s a quick quote from W. W. Sawyer’s Mathematician’s Delight:

If you cannot see what the exact speed is, begin to ask questions. Silly ones are the best to begin with. Is the speed a million miles an hour? Or one inch a century? Somewhere between these limits. Good. We now know something about the speed. Begin to bring the limits in, and see how close together they can be brought.

Study your own methods of thought. How do you know that the speed is less than a million miles an hour? What method, in fact, are you unconsciously using to estimate speed? Can this method be applied to get closer estimates?

You know what speed is. You would not believe a man who claimed to walk at 5 miles an hour, but took 3 hours to walk 6 miles. You have only to apply the same common sense to stones rolling down hillsides, and the calculus is at your command.

Don’t miss any of *“Let’s Play Math!”*: Subscribe in a reader, or get updates by Email.

Happy 11/12/13, otherwise known as “tenty-one, tenty-two, tenty-three.”

Do your young children have trouble counting in the teens? Try making up Funny Numbers to help them! It’s a great habit to develop, because Funny Numbers will come in handy as mental math tools throughout their school math career.

If you’d like to make your own Happy Math Day post, check out the instructions here: Every Day Is Mathematics Day. And please share a link in the comments section below — I’d love to see what math holiday you invent!

**Update:** The numbers 11, 12, and 13 form an arithmetic progression. If that sounds too scary for your kids, check out Patrick’s bedtime math discussion Making Progress, Arithmetically.

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Oh, my! Ben Orlin over at Math with Bad Drawings just published my new favorite math proof ever:

I had a fight with Euclid on the nature of the primes.

It got a little heated – you know how the tension climbs.It started out most civil, with a honeyed cup of tea;

we traded tales of scholars, like Descartes and Ptolemy.

But as the tea began to cool, our chatter did as well.

We’d had our fill of gossip. We sat silent for a spell.

That’s when Euclid turned to me, and said, “Hear this, my friend:

did you know the primes go on forever, with no end?” …Click here to read the whole post at Math with Bad Drawings.

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- David Coffey’s What’s the point?

Make your own “Happy Math Day” sign:

Here’s a fun activity for any age that will encourage your children to play with numbers:

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