Principle of Virtual Work in Engineering Statics | BSc mathematics Mechanics lecture 4

Four equal heavy uniform rods are freely jointed so as to form a rhombus which is freely suspended by one angular point, and the middle points of the two upper rods are connected by a light rod so that the rhombus cannot collapse. Prove that the thrust of this light rod is 4W tan(alpha ) where W is the weight of each rod and (2alpha) is the angle of the rhombus: