Let the center of the small circle be O, and the point where circle O is tangent to the circle with diameter AC be D, and let the radius of circle O be r. Lastly, let the midpoints of AB and BC be points M and N, respectively.
Since B is the center of a semicircle, BD must also be the same length as AB (because they are the radius) = 2.
MO = 1 + r, and MB = 1. OMB forms a right triangle with angel OBM = 90 degrees, since O lies on the perpendicular bisector of AC.
This means that OB, by the pythagorean theorem, has length √(1+r)2−1=√r2+2r.
Additionally, DO = r, and lines on the same line as OB. Recalling that DB = 2, we can set an equation: DO + OB = 2:
r+√r2+2r=2
√r2+2r=2−r
r2+2r=(2−r)2=r2−4r+4
2r=−4r+4
6r=4
r=23
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