12.3.2.4 Geometric

Condition: on the diameter of the km of the circle of the radius \ (r \) the point \ (a \), removed from the center of the circle to a distance \ (a \ in [0; r) \). Draw through a point \ (a \) the chord \ (l n \) so that the area of the quadrangle \ (k l m n \) is maximum. Find this square.