Finding an unknown angle

ABC is a triangle. The dynamic example calculates the size of <A. Check each box in the example to reveal that line of the working. Click and drag any blue point to change the shape of the triangle.

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1. What reason could be given to justify line (1) of the working?
2. Explain the step taken in working from line (2) to line (3).
3. Explain the operation indicated in line (4).

State of New South Wales, Department of Education and Training 2007, Created with GeoGebra