I’ve been enjoying your math-focused website. I ran across this explanation for triangle construction given an angle “A”, an opposite side “a” and a height (at “A”) of “h”. I’m trying to follow the instructions but keep getting stuck at the first part. I just can’t figure out how to “Construct an arc (part of a circle) subtending segment BC = a such that for every point A on it, angle BAC equals the given angle A” How do I choose the radius of the arc or its starting point?

Any hints?

Insructions from the site:

• A, a, ha
  

Construct an arc (part of a circle) subtending segment BC = a such that for every point A on it, angle BAC equals the given angle A. Draw a line parallel to BC at the distance ha. There could be 0,1, or 2 points of intersection of the line with the arc. In the latter case solutions are mirror images of each other; so even then there is at most one solution.

R/a = R'/a'

for radius R.