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What is the area of this quadrilateral, in cm$^2$?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
Which has the greatest area, a circle or a square, inscribed in an isosceles right angle triangle?
Cut off three right angled isosceles triangles to produce a pentagon. With two lines, cut the pentagon into three parts which can be rearranged into another square.
A square of area 40 square cms is inscribed in a semicircle. Find the area of the square that could be inscribed in a circle of the same radius.