The area of the octagon is the area of the original square minus the area of the four triangles.
Area of the original square 20 • 20 = 400
1
Area of one triangle ––– • 4 • 4 = 8
2
Area of all four triangles 4 • 8 = 32
Area of octagon (square minus triangles) 400 – 32 = 368
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