Monday, November 17, 2008

Be fruitful and multiply....

The girls are watching Schoolhouse Rock (which is one of the best things ever invented, IMHO), and I was reminded of this fun, easy math fact...

When I was a kid (okay, admittedly even now) I was never good with multiplication. Nowadays, students are encouraged to use calculators, but I still think it's important to know how to do math in your head (even if it's only basic math). The "nines" times table was always particularly hard for me. If only I'd known about these two magic little words:

Digit Sum

In the "nines" times table, one easy way to know if you're doing your math correctly is that the numbers of the digit sum (meaning the answer) of a product of nine, when added together, will ALWAYS equal 9.


9 X 2 = 18 (1 + 8 = 9)
9 X 3 = 27 (2 + 7 = 9)
9 X 4 = 36 (3 + 6 = 9)

It even works with larger numbers (obviously it gets a little more complicated, you need to be able to do somewhat more abstract math, but it does still work):

43 X 9 = 387 (3 + 8 + 7 = 18; 1 + 8 = 9)
72 X 9 = 648 (6 + 4 + 8 = 18; 1 + 8 = 9)

Even really large numbers:

94256 X 9 = 848304 (8 + 4 + 8 + 3 + 0 + 4 = 27; 2 + 7 = 9)

Obviously you're not going to normally be multiplying numbers quite that large in your head...but it's nice to know how to check your math easily.

Okay, so God was referring to a different kind of "multiplying". I came up with the title, liked it, and stuck with it. So sue me.

Why yes, I AM a dork. Thanks for noticing!


Shelly said...

that made me laugh. ha!
that would be really useful if i believed in doing math--i'm more of a guesstimator =) my husband has a quick math mind so i've retired that part of my brain--i find everything else seems to work better when that particular part of my brain is off--it works for me =)

[mr joan croft] said...

Heh, ahhh the good ol' days of Schoolhouse Rock. That and Sesame Street were the best, most educational parts of my early childhood.

I appreciated your comment. And I'm glad you responded in a more responsible manner :)
I added you to my "Those who have commented" blogroll, so I'll be sure to check back :)

Have a nice day!

Destini said...

I totally thought you were going to tell me that you and Ande were being fruitful and multiplying again! ;)

There is a neat little finger trick that works with 9's too.
If you are multiplying let's just say 7 X 9....holding your 10 fingers up(assuming you have 10), you would fold down your 7 finger. The fingers to the left of your 7 finger would be your 10's digit answer and the ones to the right would be the one's digit answer....

Devin said...

That is NEAT! I am pretty sure that I knew that once, but had forgotten. :-) Pretty cool learning about it all over again.

And, I must be a dork too, then, 'cause I liked the title!

Mom2-2goodboys said...

You know, I just recently found this out too! Why didn't they teach us this when we were in school? Hmmmmm why? I ask! It would have made multiply by nines much easier for me....