To make an arc which is tangent to 2 existing lines:
1. Select the Chamfer tool
2. Select 2-point drawing method (Use the 3-point tool if there are many lines near the two intended tangents)
3. Uncheck TRIM in the Location Bar
4. Type a Radius measurement into the Location Bar.
If you do not know the appropriate Radius -
a) Draw a line connecting the two original lines starting at about the location where you want the arc or circle to be tangent
b) SHOW INFO for the connecting line, then Select and copy the length dimension
c) Delete the measuring line
d) Paste the dimension into the RADIUS box in the Location Bar
e) Divide the length by 2 to get a radius. (Type "/2" after the dimension pasted into the RADIUS)
5. Click on the first line, drag to the second line and release
To make a circle which is tangent to 2 lines: Do steps 1 to 5 above
6. Select the circle tool
7. Select the center point modifier
8. Move the cursor over the tangent arc until the circle center point appears
9. Click and drag until the circle appears, then drag back to the arc until the tangent indicator appears
10. Release the mouse
11. Click on the circle and choose MOVE TO BACK from the ARRANGE menu
12. Click on the arc and hit DELETE