There are two methods.  The answers look different but are
equivalent.  I'll do it both ways:


If you have studied the law of cosines then:









If you haven't studied the law of cosines, it can be done another way,
but it involves the angle  instead of .

For doing it that way, draw in this green median, which, since the triangle
is isoceles, is the bisector of the central angle as well as the 
perpendicular bisector of the chord, forming two congruent right triangles.



Looking at the upper right triangle only, we see that



Multiply both sides by 10



Multiply both sides by 2



Edwin