A dynamic representation of the [i]Theorem of three perpendiculars[/i]:
Given a plane [math]\alpha[/math] and a line [i]r [/i]perpendicular to it in a point [i]P[/i], if [i]t [/i] is a line on [math]\alpha[/math] that doesn't pass through [i]P[/i] and [i]s[/i] is the perpendicular to [i]t[/i] through [i]P[/i], then line [i]t[/i] is perpendicular to the plane generated by [i]r[/i] and [i]s[/i].
