Let ABCD be a quadrilateral. Suppose that E∈ AB, F∈ BC, G∈ CD, H∈ AD are midpoints of AB, BC, CD and AD, respectively.
b
If the areas of △EBF, △FCG, △GDH and △HAE are equal to S1, S2, S3, and S4, respectively, find the area of EFGH in terms of S1, S2, S3, and S4.