Before revision:  In the present paper , those equations are successfully replaced by approximate ones, whose solutions are given explicitly in terms of elementary functions, namely , piece-wisely,  generalized logistic functions: they {\em   unveil a useful feature, that in fact  is also owned by the (numerical) solutions of the exact equations {\em  {\em   .
After revision:  In the present paper , those equations are successfully replaced by approximate ones, whose solutions are given explicitly in terms of elementary functions, namely , piece-wisely, generalized logistic functions: they {\em unveil a useful feature, that in fact is also owned by the (numerical) solutions of the exact equations {\em equations. The novelties in the work are: the way the approximate equations are obtained, using simple, analytic geometry considerations; the easy and practical formulation of the final approximate solutions; the mentioned useful feature, never disclosed before. The work's method and result prove to be robust over a range of values of the well known non-dimensional parameter called{\em .