Theorem 5.4b: The area of the region in the figure above is

 sjsj-1 sin(360(j-1)/n)

proof: This result follows from Theorem 5.1a. In this case the angle between the two sides is known to be 360(j-1)/n from Theorem 4.1. The one half is canceled out by the fact that there are two congruent triangles making up this region.

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