So BC=AC=CD
∴∠CBD=∠D,∠BAC=∠ABC
And because ∠ CBD+∠ D+∠ BAC+∠ ABC = 180.
∴∠CBD+∠ABC=90
Namely: ∠ Abe = 90.
∴ AE is the diameter ⊙ O.
(2) Shadow area = semicircle area-triangle area ABC =12.5 ∏-1/2 * 4 * 2 root number 2 1= 12.5∏-4 root number 2 1.