This Demonstration constructs a triangle given the length of its base , the length of the altitude from to and the difference between the angles at and at . This construction is an alternative to the one from Plemelj's teacher's textbook. See The Plemelj Construction of a Triangle: 2.
Step 1: Draw a line segment of length . Draw the line segment of length perpendicular to . Let be the midpoint of .
Step 2: Construct a circle such that the chord subtends an angle from points on above the chord. Let be the center of . The corresponding central angle is .
Step 3: Let be the intersection of and the line through parallel to .
Step 4: The triangle meets the stated conditions.
By construction, .
Let be the midpoint of . By construction, , so and the altitude from to has length .
It remains to prove .
The angle , since and triangle is isosceles.
By construction, the chord subtends the angle from any point above the chord.