[tex]U_1+\frac{U_1}{R_1}*R_w=U_2+\frac{U_2}{R_2}*R_w \\ \\U_1-U_2=\frac{U_2}{R_2}*R_w-\frac{U_1}{R_1}*R_w\\ \\U_1-U_2=(\frac{U_2}{R_2}-\frac{U_1}{R_1})*R_w\\ \\R_w=\frac{U_1-U_2}{\frac{U_2}{R_2}-\frac{U_1}{R_1}}=\frac{U_1-U_2}{\frac{U_2*R_1-U_1R_2}{R_1*R_2}}=\frac{(U_1-U_2)*R_1*R_2}{U_2*R_1-U_1*R_2}[/tex]
