I say the figure ABCDE is a regular pentagon.
Consider a regular decagon AD'BE'CA'DB'EC' inscribed in a circle O with radius AO. Set AO = r = 1.
• We may consider the inscribed decagon given without having a construction.
A decagon has 10 vertices and a pentagon has 5 vertices, so if we can construct the first then we can construct the second.
Connect every vertex of the decagon to every other vertex.
• This is called a complete graph.
Remove most of the lines from the last figure, for simplicity. Find the intersection of DD' and A'E; call this N.
Since the complete circle has an angle of 2p, the angle connecting adjacent vertices (such as A'OD) must be 2p / 10 = p / 5. Call this ? ("theta").
• We discuss angles in radians. The angle ? = p / 5 radians = 36°.
Since A'O and DO are both radii of circle O they are equal and triangle A'OD must be isosceles in O.
Since the sum of all angles in every triangle is p = 5?and A'OD = p / 5 = ?, DA'O = A'DO = 2p / 5 = 2?.
• Equal angles inscribed in a circle subtend equal chords.
Arc AE = Arc ED, so AA'E = EA'D = ( DA'O) / 2 = ?.
Since EA'D = NA'D = ? and A'DN = A'DO = 2?, A'ND = 2? and so triangle A'DN is isosceles in A'; and A'N = DA'.
Since NA'D = A'OD, A'DN = A'DO, and A'ND = DA'O; therefore triangles A'OD and A'DN are similar (AAA) and the ratios of corresponding sides are equal.
So DO / A'N = DA' / ND = A'O / DA'.
Also DO / DA' = A'O / DA' = NA' / ND = DA' / ND.
Since NA'D = ? and DA'O = 2?, NA'O = ?. Thus triangle ONA' is isosceles in N; and A'N = NO. Also A'NO = 3?. By (P 09), A'N = DA' = NO; call this length g.
By (P 01), DO = 1. By (P 10) and (P 11), DO / A'N = DO / NO = 1 / g.
Also DO / NO = DO / DA' = NA' / ND = NO / ND = 1 / g.
The conclusions of (P 05) through (P 12) and later (P 16) are summarized in the figures above. The angles are ?, 2?, and 3?. The line lengths are 1, g, and 1 - g.
By (P 12), DO / NO = NO / ND. But DO = NO + ND. So by substitution:
For convenience, let's rewrite this as:
Divide numerator and denominator of the left side by t:
This is a quadratic equation, so put in standard form:
So a = 1, b = -1, and c = -1; using the quadratic formula:
The ratio NO / ND is positive, so we prefer the positive root:
Since A'O = EO, triangle EOA' is isosceles in O. By (P 11), NA'O = EA'O = OEA' = ? Again, since the sum of angles in a triangle must be p = 5? A'OE = 3? Therefore triangles A'OE and ONA' are similar (AAA); A'O / EA' = NA' / EO and A'O / NA' = EA' / EO.
In (P 14) we found:
Now it is time to look again at our construction:
Since A'O = 1 and MO = 1 / 2, then by Pythagoras:
Since the radius of circle M is 1 / 2 (Step 2, 3), then OM = PM = QM = 1 / 2.
Since PA' = MA' + PM, then by (P 20, 21):
Similarly, QA' = MA' - QM, so:
Adding zero is of course allowed:
By similar arguments we show that B and C are also vertices of a regular decagon. Since a decagon has an even number of vertices, A is also a vertex. And by (P 02), we have also found the vertices of a regular pentagon. QED.