|Let's think about this figure dynamically.|
If quadrangle ABCD is given, quadrangle EFGH is decided.
If quadrangle ABCD changes, quadrangle EFGH will change.
In a word, quadrangle EFGH is a function of quadrangle ABCD.
Various problems and concepts are developed by introducing the view of "Function" to a figure like the above.
And, the DGS is the best way to think in the same as above.
Of course, there are some problems that can be analyzed as other functions.
This "quadrangle¨quadrangle" can be considerd as an introduction to such problems.