Attaching a line to an arc or circle can be automatically snapped with a SOLIDWORKS tangent relation if you hover over the tangent location for a bit.
Starting the line from an arc or circle may not add the tangent relation automatically. However you can hover over the arc to ‘wake up’ the center point, start the line on the arc and drag the line out in the tangent direction. You can even spin it around the curve and it will remain tangent.
NOTE: This method works for Circles and Arcs as they have center points to wake up. You can start a line from a Spline and wait for the inference lines to snap tangentially, but it won’t follow the Spline with a tangent relation as shown in the video to snap the other end of the line.
To see this in action, here’s a video demonstration:
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