1981 USAMO Problems/Problem 1
Prove that if is not a multiple of , then the angle can be trisected with ruler and compasses.
Let . Multiply throughout by . We get
Re-arranging, we get
A way to interpret it is that if we know the value , then the remainder angle of subtracting times the given angle from gives us , the desired trisected angle.
This can be extended to the case when where now, the equation becomes
Hence in this case, we will have to subtract times the original angle from to get twice the the trisected angle. We can bisect it after that to get the trisected angle.
If regular polygons of sides and sides can be constructed, where and are relatively prime integers greater than or equal to three, then regular polygons of sides can be constructed. Indeed, such a polygon can be constructed by first constructing an -gon, and then creating distinct -gons with at least one vertex being a vertex of the -gon.
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