Wikipedia:Articles for deletion/Distance from a point to a line
WP:NOTTEXTBOOK states "''The purpose of Wikipedia is to present facts, not to teach subject matter. It is not appropriate to create or edit articles that read as textbooks, with leading questions and systematic problem solutions as examples.''"  The topic is not presented in this way and, even if it were, the issue would be best addressed by editing not deletion, per 
— User:Colonel, 12:10, 9 March 2013 (UTC)
Stance: keep
Cited policy: WP:PRESERVE (Wikipedia:Editing policy)