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The pentagon could be seen as a multiple of 5 triangles, they are exactly the same.
Area of a triangle is apothem x the lenght of the pentagon side / 2.
The area of the whole pentagon is apothem x 5 times the lenght of the pentagon side / 2 = apothem x perimeter / 2 = 20 cm x 145,3 cm / 2 = 1,453 cm².
