Use the [icon]/images/ggb/toolbar/mode_orthogonalthreed.png[/icon] tool to draw a line that is perpendicular to the given plane at point A.[br]Then use the [icon]/images/ggb/toolbar/mode_point.png[/icon] tool to place one point (B) on the line and another point © on the given plane.[br]Now use the [icon]/images/ggb/toolbar/mode_segment.png[/icon] tool to draw a line in the plane that goes through point A and the point (B) you added to the plane.[br][br]To verify that the perpendicular line that you made earlier does form a right angle like a perpendicular line should, use the [icon]/images/ggb/toolbar/mode_angle.png[/icon] to measure [math]\angle[/math][i]BAC. [/i]If you need to, use the [icon]/images/ggb/toolbar/mode_move.png[/icon] tool to move the name of point A and the angle measure around in the diagram so that you can read them clearly.