This article describes the construction of a regular pentagon.
Also explained is why this construction is correct.
Please look at figure 1.
ABCDE is the pentagon.
The construction involves the following steps:
2. construct center M of CD
3. construct line perpendicular to CD and through D
4. draw N, so DN = DM
5. extend line CN
6. draw circle with center N and radius DN, P is intersection with extended line from 5.
7. extend perpendicular bisector of CD
8. draw circle with center C and radius CP, A is intersection with bisector of CD
9. draw circles with radius CD and centers A, C and D. Points B and E are other angles of pentagon.
Why is this correct?
Look at figure 2:
Each angle of the pentagon is 108 degrees.
All angles of the triangles together are 5 * 180 = 900 degrees.
Subtract the angles in the center: 900 - 360 = 540 graden.
Each angle of the pentagon is 540 / 5 = 108 degrees.
LECD = LCED so
LECD = (180 - 108) / 2 = 36 degrees.
Each angle marked + 36 degrees.
CS is bisector of LACD
LCSD = LSDC = 72 degrees, so
We apply this lemma:
having the same ratio as the sides of the angle
x2 - x - 1 = 0
than the construction is correct.
Calculation is left to the reader.