Make a circle with OA as the radius, extend OA to find a point C, use CA as the radius to make an arc intersect with E and F, and then use CA as the diagonal for square ABCD. If E, O, and F are collinear, AB=10, find the yellow part area =?
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Connect EF, AE, AF, you can get OE=OF=OA, AE=AF=√2OA, then
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⇒
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and
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