Let [color=#c51414] [i]P[/i][/color] be a point on a plane [color=#555][math]\alpha[/math][/color]. [br]Draw the line [i]r[/i], perpendicular to plane [color=#555][math]\alpha[/math][/color] in [i][color=#c51414]P[/color][/i], then a new line [color=#c51414][i]t[/i][/color] in the same plane.[br]Now draw the line [color=#1551b5][i]s[/i][/color], perpendicular to [color=c51414][i]t[/i][/color] through [i][color=#c51414]P[/color][/i].[br]Draw the plane [color=#1551b5][math]\beta[/math][/color], through [i]r[/i] and [color=#1551b5][i]s[/i][/color].[br][br][math]\Longrightarrow[/math] the plane[color=#1551b5] [math] \beta[/math][/color] is perpendicular to line [color=c51414][i]t[/i][/color].[br][br]Move the points in the construction and explore the theorem.