

Construction of a Regular Pentagon 


Introduction
This article describes the construction of a regular pentagon.
Also explained is why this construction is correct.
The construction
Please look at figure 1.
ABCDE is the pentagon.
 fig. 1 
The construction involves the following steps:
1. choose the length of line CD, the base of the pentagon
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:
 fig. 2 
Each angle of the pentagon is 108 degrees.
The pentagon may be dissected into 5 equal triangles (with 1 angle in the center)
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.
Triangle CDE is isosceles, so
LECD = LCED so
Each angle marked + 36 degrees.
CS is bisector of LACD
LCSD = LSDC = 72 degrees, so
Say the length of CD = 1.
We apply this lemma:
In a triangle, the bisector of an angle divides the opposite side in parts
having the same ratio as the sides of the angle
So:
If AC = x we get :
x : 1 = 1 : ( x  1 ) ...........or
x^{2}  x  1 = 0
the ABC rule:
x =  
If we can prove that CP = x (see figure 1) than the construction is correct.
Calculation is left to the reader.

