A line tangent to a circle is one that intersects the circle at exactly one point and does not cross through the interior of the circle. To draw a tangent line to circle O above, go to the fourth button from the left on the menu bar and choose Tangents. Touch or click on the circle to choose it, then touch or click on point A as the point of tangency. An important relationship exists between the line tangent to a circle at a point and the radius of the circle whose endpoint is the point of tangency. Use the Segment tool (third button from the left, choose Segment) to draw radius OA. Next, go to the arrow (the first button) and select Move. Take point A and move it around the circle. Both the tangent line and radius will move with the point.

Check your hypothesis by measuring the angle. Go to the eighth button from the left and choose Angles. Touch or click on the tangent line, then touch or click on the radius to measure the angle between them. Again, use the Move tool to move point A along the circle. Observe that the angle measure does not change.