BAC is the given angle and r is the fixed halfline with D as origin.
Build the angle BAC , using Angle.
Build the halfline, using Halfline.
Then, draw a circle, using Circle with center point, centering in A and with radius length less than the AB distance.
On the halfline, build a circle equal to the previous, using Circle with pt two pts, clicking on D, A, F (the intersection between the angle and the first circle).

Build another circle using Circle with center point, clicking on F and the intersection between the segment AC and Circle1.
Copy this circle on the halfline using Circle with pt two pts, clicking on G, F and H.
Now, build the desired angle, using Angle, clicking on D, G and I.
The angle GAI is equal to the given angle BAC, infact the isosceles triangles GAI, FAH, are equal because they have, by construction, the three sides ordinally equal.

With the construction described above, it is possible to determine the sum of several angles,
or the multiple of a given angle, according to a fixed number, or the difference between two angles.

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