How to draw a keypoint arc tangent to a point on an existing curve, or concentric to an existing arc.
Draw an arc tangent to a curve
.

Draw an arc concentric to another arc
.

Concentric arcs are usually drawn in the direction that will create the smallest sweep angle.
To
draw an arc in the opposite direction, choose Windows > Information > Information Window
. Open the Attributes section
and click Complement.
Choose Windows > Information > Information Window
. Set the Angle/Sweep attribute
to 360.