A Visual Approach to Simplifying Radicals (A Get Out of Jail Free Card)

The radical sign is like a prison.Â Twelve can be expressed as a product of prime factors so âˆš12 =Â âˆš(2Ã—2Ã—3). The 2’s pair up and try to break out. Sadly, only one of them survives the escape.Â âˆš12 becomes 2âˆš3.

That’s how I was taught to simplify radicals. No joke.

I imagined the numbers yelling “All in the name of liberty! Got to be free! JAlLBREAK!” as they scaled the prison walls. To this day, I can’t getÂ this songÂ out of my head when I teach this topic.

Many students are shown this method, albeit without the prison imagery. Write the prime factorization of the number. Circle the pairs. Write/multiply circled numbers outside the radical sign. There is real math behind this procedure. By definition,Â âˆš2Â Ã—Â âˆš2 = 2. However, I found that students who were taught this method couldn’t tell me whyÂ âˆš(2Ã—2Ã—3) =Â 2âˆš3. Where did the other 2 go?

Instead, I asked students to evaluateÂ âˆš12, then 2âˆš3, using their calculators. Why are they equivalent? Students factored âˆš12 asÂ âˆš4 Ã— âˆš3 (with some scaffolding for some). They understood where the 2 came from. Some began by factoringÂ âˆš12 asÂ âˆš6Â Ã— âˆš2. Correct, but not helpful. The importance of finding factors that are perfect squares was discussed.

Marc Garneau shared with me his visual approach to simplifying radicals.

Consider a square with an area of 24. The side has lengthÂ âˆš24.

This square can be divided into 4 smaller squares, each with an area of 6. The sides of these smaller squares have length âˆš6. Two of these lengths make up the side length of the large square, soÂ âˆš24 = 2âˆš6.

24 can also be divided into 3 rectangles, each with an area of 8. Again, correct, but not helpful. How to simplifyÂ âˆš45 as 3âˆš5 andÂ âˆš72 as 6âˆš2 are also shown above. Again, factors that are perfect squares are key.

I think it would be interesting to try this out. Some students may prefer this method, but most students will likely move towards simplifying radicals without drawing pictures. But by drawing pictures as they are learning this skill, students will be connecting mathematical ideas and building conceptual understanding. New learning (simplifying radicals in Math 10) will be connected to prior learning (concept of a square root introduced in Math 8). Students will have a more solid understanding of why perfect squares are used.

Turn it Around

In More Good Questions: Great Ways to Differentiate Secondary Mathematics Instruction, Dr. Marian Small discusses the turn-around strategy to create open questions.

Instead of asking “The legs of a right triangle are 3 cm and 6 cm long. What is the hypotenuse?” the teacher can ask “The answer isÂ âˆš45. What could the question be?”

There are many possible questions. For example,

Determine the length of the hypotenuse.

Determine the length of x.

A square has an area of 45 cmÂ². What is the side length?

What is an example of a square root that has a value between 6 and 7?

Which number is the greatest:Â âˆš37, 6, 6Â½,Â âˆš45?

Students will come up with a variety of questions. However, at first, I imagine the response to open questions such asÂ “The answer isÂ âˆš45. What could the question be?” will be silence. Students are used to being asked questions where there is one correct answer. In math, you either get it or you don’t. It’s not just questions that need turning around. This black and white view of mathematics also needs turning around. With time and practice, class discussions about open questions can help change this attitude.

“Under the Mâ€¦ the square root of 12”

On this blog, sometimes I share my thoughts about transforming math education. This is not one of those times.

Here, I’m using my blog as a digital filing cabinet.

One activity that my students enjoyed was MATHO (and its variations FACTO and TRIGO).

Have students select and place answers from the bottom of each column to fill up their MATHO cards. In some versions, I pulled prepared questions from a hat. In other versions, I translated answers to questions on the fly. For example, if I grabbed 2âˆš3, I called out “Under the Mâ€¦ the square root of 12”. After a student shouts “MATHO!” ask potential winners to read aloud their numbers. (Remember to keep track of answers you have called.)

Nothing revolutionary here â€“ just a fun way to review content.

By the way, if you are looking to read about changing things, please check out Sam Shah’s recent post,Â The Messiness of Trying Something New.