- Draw a circle.
- Draw a square box.
- Select snap to shapes.
- With the straight line tool, start a line approximately in the centre of the circle and extend it to the require box corner - this will 'snap'.
- Release the line.
- Click on the start of the line and move it about until it 'snaps' on the centre.
- Release the line.
Acorn
P.S. Changing the centre to a tangent is much more challenging.