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The exterior angle of a triangle

ABC is a triangle. BC is produced to D. Click and drag any of the blue points to change the shape and orientation of the triangle.

1. What relationship exists between <ACD and the sum of <A and <B?
2. Complete the following: "The exterior angle of a triangle is equal to ..."
3. What relationship exists between <ACD and <ACB? How can you use this result to prove (2) above?

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