Question 3915
 This question is somehow very unclear ,it did not show what the given
 conditions are and what you ask for ???

 I think it should be given the length of a chord on a circle. Then try
 to find the distance from its mid point to the circumference of the circle ?

                P
                 | M
           ------|-------
          A      |       B
                 |
                 |
                 | O
  Here, AB is the chord,O is the center, P is on the circumference of
  the circle and M is the midpoint of AB.

  We see that OB^2 = MB^2 + OM^2, let AB = L, OB = r.
  We have OM = sqrt(r^2 - 1/4 L^2) 
  and so PM = OP - OM = r - sqrt(r^2 - 1/4 L^2).
 
  Also, I don't think any further explanations are necessary. 
  Try to figure out the details if you have trouble understanding.

  Kenny