Constructing a Parallel Line through an Inaccessible Point
Given a straight line and two lines and that intersect at an inaccessible point , construct a straight line through parallel to . You can drag or .[more]
The general method for solving this problem is to use a transformation so that the inaccessible point becomes the accessible point . In this case, the transformation is a translation in the direction of .
Using the slider, translate and parallel to to and such that the corresponding translates of and intersect at the accessible point .
Then draw the line through parallel to .[less]
 B. I. Argunov and M. B. Balk, Elementary Geometry (in Russian), Moscow: Prosveščenie, 1966 pp. 338–339.