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I can't see how the textbook produces the following relationships between angles:
$$ \theta = \phi + \alpha \qquad (1)$$
$$ 2\theta = \alpha + \alpha ' \qquad (2)$$
My thinking is that the exterior angle theorem for triangles was used to create expression ##(1)##, but I am unsure as to how expression ##(2)## came about. Was the exterior angle theorem simply used twice to produce both equations?
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