Three segments make a right-angled triangle:

1)  the half of the chord;

2)  the perpendicular bisector from the center of the circle to the chord;  and

3)  the radius drawn from the center to the chord's endpoint.


The half of the chord has the length of  = 4 units.

The perpendicular bisector from the center of the circle to the chord has the length

     =  = .

So, the right-angled triangle has two legs of 4 units and  units long.


The radius of the circle is the hypotenuse of this triangle.

Hence, its length is  =  = .