This equation is quadratic in Xn (and in On). We note that of the two solutions to this equation, only one of them will be a solution of the original equation as the second has been in traduced due to squaring. The solutions are complicated functions of a and b. We denote the one solution of the original equation by Xn ¼ L2ðOnÞ and the graph of this solution for a ¼ 0:5, b ¼ 0:9 is shown in Fig. 16. It is easily verified that the region between the lines Xn ¼ L1ðOnÞ and Xn ¼ L2ðOnÞ maps onto the intersection of the line (80) with region(c)of Fig. 7.