At the start of the lesson students recap enlarging shapes to a positive scale factor by matching a description with the correct enlargement. To save time I provide handouts for the class so students can draw rays to find the centre. This typically takes about 5 minutes. I then feedback the matching pairs to address any misconceptions.
To address these misconceptions I explain that a negative scale factor changes the direction of the ray so it travels from the vertices on the object through the centre.
I use the interactive Geogebra applet to help students visualise the difference between a positive and negative scale factor.
Next, I demonstrate how to perform a negative scale factor enlargement using rays drawn from the object through the centre.
Click here to watch the video on YouTube.
When students are ready to work independently I ask them to complete the enlargements shown below. This is printed on the back of the handout the class received for the starter.
As the lesson progresses I use mini-plenaries to feedback the enlargements. When this is completed students attempt more challenging questions from the worksheet.
The final activity challenges students to describe two enlargements by a negative scale factor. The first enlargement involves a scale factor of -0.5 and the second -2.
My name is Jonathan Robinson and I am passionate about teaching mathematics. I am currently Head of Maths in the South East of England and have been teaching for over 15 years. I am proud to have helped teachers all over the world to continue to engage and inspire their students with my lessons.
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