正八边形面积 = 8 x S(△OAB)
= 8 x (OB x AC / 2)
= 8 x (OA x AC / 2)
= 8 x [√2 x (8/2)] x (8/2) / 2)
= 8 x √2 x 4 x 4 / 2
= 64√2
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