Quadrilateral **ABCD **is an isosceles trapezoid, with bases **AB **and **CD**. A circle is inscribed in the trapezoid, as shown below. (In other words, the circle is tangent to all the sides of the trapezoid.) The length of base **AB **is **2x **and the length of base **CD **is **2y**. Prove that the radius of the inscribed circle is √**xy **.