Friday, March 5, 2010

Multiplication approaches

When I started volunteering in my fifth grade class, we had an exercise that says "Solve this equation in two different ways." I panicked because I had ever only learned one way - the standard algorithm for multiplication.

Luckily, I've been working on enough multiplication tables now that I have a variety of ways to solve multiplication problems! It's great, my students have taught me a thing or two. I'll go over some different ways of multiplying that don't require a good knowledge of DDR.

1) Adding
Yes, it's not the preferred method of multiplication at the 5th grade level, but every once in a while, it may work. I would suggest you convince your students that it's not the most efficient way to do multiplication, especially if you're doing anything higher than one digit multiplication.

2) Partial sums
I've touched on this in my Greg Tang posts, but we can work through this method here. I actually really like this method and I think it works well for smaller two digit multiplication.

To do partial sums, you use several "easy" products and sum them all up to get to the answer. In the case of 83 x 24:

83 x 10 = 830
83 x 10 = 830
83 x 4 = 332

830 + 830 + 332 = 1992

Easy, no?

3) The array
I actually really enjoy this method as well. It's a more involved method of partial sums, but the visual aspect of it helps some students more than others.

You set up by making a box and separating the tens and ones. Multiply through and put your answers in the boxes. Then add everything up.

Make sure you get your students to check their zeros when multiplying the tens. Ask them how many zeros you start with and how many are in the answer. It will help with their self checking later on.

So there you have it, four methods to solving a multiplication problem!

On a side note: leave it to the fifth graders to call me out on my doofy haircut. And I got the most adorable chorus of "Hi Ms. Minh" again when I came into the 3rd grade class. They. are. so. cute. I don't think it'll wear off anytime soon. I hope it doesn't.


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