[tex]\frac{1}{x_2}-\frac{1}{x_1}=\frac{1}{f} (1)\\ \frac{1}{x'_2}-\frac{1}{x_1}=\frac{1}{f'} (2)\\(2)-(1)=> \frac{1}{x'_2}-\frac{1}{x_2}=\frac{1}{f'}-\frac{1}{f}\\\frac{1}{f} =(n-1)(\frac{1}{R_1}-\frac{1}{R_2}) (3)\\\frac{1}{f'} =(\frac{n}{n_a} -1)(\frac{1}{R_1}-\frac{1}{R_2})(4)\\ (4):(3)=>\frac{f'}{f}=\frac{(n-1)n_a}{n-n_a} =>f'=\frac{(n-1)n_af}{n-n_a}\\\frac{1}{x_2}-\frac{1}{x'_2} =\frac{1}{f} -\frac{1}{f'} =\frac{1}{f} -\frac{1}{\frac{(n-1)n_af}{n-n_a}}=\frac{1}{f}*\frac{n(n_a-1)}{(n-1)n_a}[/tex]
[tex]f=\frac{(n_a-1)nx_2x'_2}{(n-1)n_a(x'_2-x_2)}[/tex]
