How do you find the radius of a partial circle?

Answer 1

Assuming you mean measure it:

One way I can think of is:

1. find the centre of the partial circle;
2. draw a radius to one end of the partial circle;
3. measure it.

To find the centre:

1. draw any two chords (not parallel to each other);
2. construct their perpendicular bisectors;
3. where these meet is the centre of the [partial] circle.

Answer 2

generally; (x-a)2 + (y-b)2 = r2

a = x coordinate of centre of circle

b = y coordinate of centre of circle

r = radius

is this what you meant?