A regular pentagon is a five-sided polygon with sides of equal length and interior angles of 108° (3π/5 rad). Because 5 is a Fermat prime, you can construct a regular pentagon using only a straightedge and compass.
Draw a line segment AB.
Draw two circles, 1 and 2, centred at A and B, both with radius AB. The points where they intersect each other are C and D.
Connect C and D.
Draw circle 3 with centre C and radius AB. Circle 3 intersects circles 1 and 2 in E and F respectively. It also intersects line segment DC in G.
Connect E and F with G. Extend these lines far enough so they intersect circles 1 and 2. Call these new intersection points H and I.
Draw circles 4 and 5 centred at H and I with radius AB. Name the upper intersection point J.
Connect A, B, I, J and H.
Trace the pentagon in ink or another colour and erase any construction lines (if needed).
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