The motion of c on arc AB does not coincide with a and B.

Then assume that x=0 when c and b coincide.

When c and a coincide, BC=AB=2.

Using the vertical diameter theorem, we can know that BC=2BD=2x.

So the root number 2 of 2x=2.

X= root number 2

So 0 < the square root of x<2.