proportion of angles 2018

load the attachment
https://www.geogebra.org/m/KrYWuNEv
see the description of the construction
slider -[TEX] alpha[/TEX] -select the angle
slider - point P - ruler with a socket, point Q must be line n

Whether you are from the previous understand how it works proportions of angles, or that you explain step by step?

https://www.geogebra.org/m/CukhmEVy

straightedge slip on the point B - line i

divider FIG , slides on straightedge , after slipping point F straighte line BC , point G describes lokus1

section lokus1 and line k point J , when changing the angle, the point J must be manually set to the intersection

construction of regular polygon is possible by means of the proportion of the angle