Three circles with radius r are drawn as shown, each with its center on the circumference of the other two circles. A, B and C are the centers of the three circles. Prove that an expression for the area of the shaded region is:

A quadrilateral RSTV is circumscribed about a circle so that its tangent sides are at the endpoints of two intersecting diameters. aWhat types of quadrilateral is RSTV? bIf the diameters are also perpendicular, what type of quadrilateral is RSTV?