Question 195018
Lets  use  a  rough  sketch  to  define  this  situation.
.
draw  a  circle.  Then  draw  a  radius  in  the  horizontal  to  right  position.
If  we  then draw  a  chord,  perpendicular  to  the  radius, about  2/3  way  out on  radius.

Now  finally  draw  a  line  from  the  center  of  the  circle,  to  where  the  chord  intercects  the  circle.
This  last  line  is  a  radius,  pls  label  "r".   It  is  also  the  hypotenuse  to  a  right  triangle,  formed  by  the  initial  horizontal  radius,  and  the perpendicular  chord.

This  right  triangle  can  be  understood  using  the  pythagorean  theorem,  c^2 = a^2 +b^2.

Generally,  this  is  all  that  is  required.

Please  remember  also  that  the total  chord  is  bisected  into  two  equal  lengths  by  the
perpendicular  radius.

Hopefully this  helps  your  ubderstanding.