The connecting line between the vertex and the midpoint of the bottom of the isosceles triangle is the height of the isosceles triangle.

Let the vertex be a, the left intersection point be b and the right intersection point be c.

(AO × BC )=5

And BC = 2×A0.

So AO equals the root number 5.

Derive from the area formula

Circular area = 5π = 15.7