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Similar triangles (part 1)

In the figure, DE is drawn parallel to BC. Click and drag any of the blue points to change the shape and orientation of the triangle ABC.

1. Given that DE is parallel to BC, what can be said of <ABC and <ADE? What other angles are equal?
2. What can be said of triangles ABC and ADE?
3. What is the effect on the ratios of corresponding sides when the point D is moved? Why is this so?
4. What is the effect on the ratios of corresponding sides when either of the points A, B and C are moved? Why is this so?

© State of New South Wales, Department of Education, 2018