Method A

  1. Drag a tangent line from the tool bar and pause over the perimeter of the circle. Wait for the popup to show you attachment points and then select the desired location. The example below shows attachment to the perimeter that was the closest to the drop point. Attaching to a grid point at the top or sides of the circle will give you a horizontal or vertical tangent line.
  2. To move the tangent location on the circle, select and move the tangent line by dragging the point (i.e., Point a in this example). The point will move freely while the line slope will change to stay tangent to the circle. You can drop the point (and associated line) back onto the circle perimeter or drop onto another location.

Interactive Geometry - Example-Tangent