[<< wikibooks] Descriptive Geometry/Mathematical Constructions/Triangle to Right Triangle
Given: Triangle 1 
Find: Triangle 2 (same area, right angle) 
Remember: Area = (base x height)/2, so triangles with the same base and height have the same area.
1. Draw a segment parallel to the base of Triangle 1 through the opposite vertex. 
2. Any triangle drawn with the same base that hits the line parallel to it will have the same area (base remains the same, height remains constant because parallel line preserves a constant distance), so draw a perpendicular line from the base to the parallel line.