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The interior angles of an octagon are 135 degrees, which means that if you extend one of the sides, the complementary external angle is 45 degrees. So imagine an octagon instead as a square with four cut out right triangles on the corners.

Let's start with the length of a side x. Since the right triangles have a hypotenuse x, we know that the legs are length (sqrt(2)/2)x. The area of the triangle is 1/2 * base * height, which means one triangle is x2 / 4. Since there are 4 triangles, the total area of the triangles is x2.

Next, we need the area of the square. One side is a leg of the octagon plus two legs of our triangles, which would be x + (sqrt(2)/2)x + (sqrt(2)/2)x = x + sqrt(2)*x. Square this, and you get 3x2 + 2*sqrt(2)*x2. But! Subtract the area of the triangles from this, and you get (after some redistributing):

A = 2x2 * (1 + sqrt(2))

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