Angles Which Can Be Trisected
However, some angles can be trisected. For example, for any angle, the angle can be trivially trisected by ignoring the given angle and directly constructing an angle of measure . There are angles which are not constructible, but are trisectible. For example, is such an angle: five copies of combine to make an angle of measure, which is a full circle plus the desired . More generally, for a positive integer, an angle of measure is trisectible if and only if does not divide ; if is a prime number, this angle is constructible if and only is a Fermat prime.
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