1. Draw 3 circles (same radius) with centers at any given coordinate points.

2. Set up ' layer ' construction and draw polyline (A) to connect all the centers.

3. Draw a line (B) > using Relative Angle ( LR) set to 90 and at an arbitrary length (say e.g. 100) and 'snap' to the center/middle of one line to produce a line perpendicular to it (90 degree right angle).

4. Repeat for the other line (C), the intersection of these two lines provides the center point (D) for the 4th circle.

5. From

**any**circle draw a line (E) from its center to the intersection, this will be the radius for the 4th circle.

Job done.