Click Circle
on the Sketch toolbar.
Sketch the circle as shown. Make sure the centerpoint of the circle is concentric with the tangent arc.
Now sketch a circle for the extruded cut through the bodies.
Click Circle
on the Sketch toolbar.
Sketch the circle as shown. Make sure the centerpoint of the circle is concentric with the tangent arc.
The pointer changes to 
when it is concentric with the arc.
Click Smart Dimension
on the Sketch toolbar,
and dimension the sketch as shown.
