Handbook - Tutorial - Problems about orthogonality and parallelism

Draw the perpendicular to a halfline
from its origin

Halfline1 is the given halfline, with origin A.
Build the halfline Halfline1, using halfline.
Build a circle Circle1 centered in a point C, extern to Halfline1, using Circle with center point, so that the radius will be CA.
The circle Circle1 will intersect Halfline1 in another point B.
Build a line CB, using Line.
E is the other intersection of CB with the circumference.
Build a line Line2, using Line, joining E with A.
This is the desired line, infact, the angle EAB is enscribed in a halfcircumference, so that it is rect.
That's why Line2 is perpendicular to Line1.