1) Use the Line tool[icon]/images/ggb/toolbar/mode_join.png[/icon] to create a line between the center of the circle and the point on the circle.[br]2) Create a line perpendicular to the one you just created through the point on the circle.[br]3) Use the Angle Measure tool[icon]/images/ggb/toolbar/mode_angle.png[/icon] to verify that this new line is perpendicular to the first one.