A Father’s Day Gift Worth Waiting For

July 2, 2014 at 6:34 am Leave a comment

Fathers DayAlex made a Father’s Day Book for me. Because the book didn’t make it on our trip to France, however, I didn’t receive it until this past weekend. It was worth the wait.

The book was laudatory in praising my handling of routine fatherly duties:

I loved when you took me to Smashburger.

I appreciated when you helped me find a worm.

I love when you read to me at night.

I love when I see you at the sign-out sheet [at after-school care]. It means I can spend time with you.

But my favorite accolade — surprise! — was mathematical:

I liked the multiplication trick you taught me. Take two numbers, find the middle [average], square it. Find the difference [from one number to the average], square it, subtract it. (BOOM! Done!)

Priceless.

The trick that I taught him was how to use the difference of squares to quickly find a product. For instance, if you want to multiply 23 × 17, then…

  • The average of 23 an 17 is 20, and 202 = 400.
  • The difference between 23 and 20 is 3, and 32 = 9.
  • Subtract 400 – 9 = 391.
  • So, 23 × 17 = 391.
  • BOOM! Done!

This works because

(a + b)(a - b) = a^2 - b^2 ,

and if you let a = 20 and b = 3, then you have

23 \times 17 = (20 + 3)(20 - 3) = 20^2 - 3^2 .

In particular, I suggested this method if (1) the numbers are relatively small and (2) either both are odd or both are even. I would not recommend this method for finding the product 6,433 × 58:

  • The average is 3,245.5, and (3,245.5)2 = 10,533,270.25.
  • The difference between 6,433 and 3,245.5 is 3,187.5, and (3,187.5)2 = 10,160,156.25.
  • Subtract 10,533,270.25 – 10,160,156.25 = 373,114.
  • So, 6,433 × 58 = 373,114.

Sure, it works, but that problem screams for a calculator. The trick only has utility when the numbers are small and nice enough that finding the square of the average and difference is reasonable.

Then again, it’s not atypical for sons to do unreasonable things…

Son: Would you do my homework?

Dad: Sorry, son, it wouldn’t be right.

Son: That’s okay. Can you give it a try, anyway?

I’m just glad that my sons understand math at an abstract level…

A young boy asks his mother for some help with math. “There are four ducks on a pond. Two more ducks join them on the pond. How many ducks are there?”

The mother is surprised. She asks, “You don’t know what 4 + 2 is?”

“Sure, I do,” says the boy. “It’s 6. But what does that have to do with ducks?”

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Math in France Judge This Joke By Its Size, Do You?

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About MJ4MF

The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

MJ4MF (offline version)

Math Jokes 4 Mathy Folks is available from Amazon, Borders, Barnes & Noble, NCTM, Robert D. Reed Publishers, and other purveyors of exceptional literature.

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