When it comes to teaching solving proportions, NOTHING BOTHERS ME MORE than when my students yell out “Cross Multiply and Divide!”
They yell it so proudly like they already know what I’m going to teach them. Like they are masters of proportions.
Where Does the Problem Start with Solving Proportions?
The problem is a good Pre-Algebra teacher would not teach them this trick because when they learn proportions in Pre-Algebra this is just an intro lesson to a much larger piece of Algebra 1.
So you have to start by wiping that from their mind and right off the bat show them a proportion where there are variables on the top of both proportions. This will allow you to say you can’t just always cross multiply and divide because you can’t just divide complicated binomials inside proportions.
Here’s where you become the savior!
Then you follow that up with these words…
“Instead of remembering this trick, that doesn’t always work, I am going to show you a simple method that will work every single time you need to find variables within proportions.”
Reiterate that this method will ALWAYS WORK!
Here’s how you teach it:
- Have them Circle the diagonals.
- Multiply them together on opposite sides of an equal sign.
- Show them how it looks like an Algebra problem they have solved before.
- Now Solve!
You should start with simple proportions showing the diagonals are equal and get progressively harder and add variables as they become more confident.
