Area of a part of a circle

Find the area of the shaded portion of each square.
The are two related examples in an applet. (See the slider)
Red points are moveable, so you can change the length of the square´s side.

Sorry, the GeoGebra Applet could not be started. Please make sure that Java 1.4.2 (or later) is installed and active in your browser (Click here to install Java now)

G a SOSPg for IMSS, 8.4.2010, Created with GeoGebra