CEMC Cayley, 1998

Geometry Difficulty 3.0 AMC 10/12 Find the answer Canada

Each side of square $ABCD$ is 8. A circle is drawn through $A$ and $D$ so that it is tangent to $BC$ . What is the radius of this circle?

(A) $4$ (B) $5$ (C) $6$ (D) $4\sqrt{2}$ (E) $5.25$

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