Available in TurboCAD LTE Pro only 
Makes an arc or circle tangent to another arc or circle, or to a line.
- Start with a line and an arc/circle. Activate the Tangent constraint and select either object (when one object is a line, the selection order does not matter).
- Select the other object.
- If you edit either object, the tangent constraint will be maintained.
. . . the line is moved so that its extension is tangent to the circle. 
. . . the arc is moved so that its extension is tangent to the arc/circle. 