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why in the symmetry energy only squared asymmetry parameter are exist and there is not the first power of asymmetry parameter?

I have a question about symmetry energy in semi-empirical mass formula, According to semi-empirical mass formula as follows: E=avA-asA2/3-acZ(Z-1)/A1/3-asym(N-Z)2/A why in the symmetry energy only squared parameter symmetry are exist and there is not the first power of asymmetry parameter?