Semicircle in a semicircle

The diagram shows two semicircular arcs... What is the diameter of the shaded region?

Problem



The diagram shows two semicircular arcs, PQRS and QOR . The diameters, PS and QR , of the two semicircles are parallel; PS is of length 4 and is a tangent to semicircular arc QOR .

What is the area of the shaded region?

Image
Semicircle in a Semicircle



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