This is a lesson where you’re going to have to do some corrective teaching. Odds are their previous teacher taught them “cross multiply and divide!”
This method of teaching ratios and proportions is no more than a trick! It shows nothing about knowing why the students are doing what they are doing. It is far more important to show them that the diagonals are equal instead. If they understand that the diagonals must be equal rather than cross multiplying and divide then they will be able to solve any proportion as they come along throughout different math classes.
Do Not Use Tricks When Teaching Ratios and Proportions
In conclusion, do all of your students future math teachers a favor and do not teach them to “cross multiply and divide”. Teach them to set the product of the diagonals equal to each other and then solve!
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