The Tangent Segment tool lets you create a segment that is tangent to a circle, either real or imaginary. We say “real or imaginary” because really what you’re doing is selecting a center and a radius and then drawing a tangent line to that imaginary circle. If there happens to be a real circle there too, then you’ll draw a tangent line to it. Your initial touch define the start of the tangent line. You then use the mark button
to define first the center of the circle and then some point on the circumference of the circle. A tangent line will appear then. You can switch which side of the circle the tangent line is drawn to by dragging your finger across the line connecting the start of the segment and the center of the circle. Here’s an example of Tangent Segment construction: