Question 754364
A Pythagorean question.
Consider a line coming from centre O
and meeting the chord AB at its centre.
The chord would be divided into two pieces
each measuring 30cm.
Now if we swing a radius round to extend
from centre O to where AB touches the circumference
of the circle.
We now have a right angled triangle.
The radius is the hypotenuse, half of AB is the 
base and the line from the centre to the chord 
is the height.
By applying Pytagoras: the sum of the two sides
squared = the hypotenuse squared.
By adjusting the formula we find that if we take 
the hypotenuse squared and take away from it,
the base squared, the answer when it is square 
rooted = the distance from the centre O to the chord AB.
         50^2 - 30^2 = the  height^2
            height^2 = 1600
            height  =  40 cm.
This is the distance from the centre O to the chord AB.
Hope this helps
:-)