[tex]\displaystyle\bf\\\frac{o-c}{c^{2}+o^{2} } \cdot\Big(\frac{c+o}{c} -\frac{2c}{c-o}\Big)= \frac{o-c}{c^{2} +o^{2} } \cdot\frac{(c+o)\cdot(c-o)-2c\cdot c}{c\cdot(c-o)} =\\\\\\=-\frac{c^{2}-o^{2} -2c^{2} }{c\cdot(c^{2} +o^{2} )} =-\frac{-c^{2}-o^{2} }{c\cdot(c^{2} +o^{2}) } =\frac{c^{2} +o^{2} }{c\cdot(c^{2} +o^{2}) } =\frac{1}{c}\\\\\\c=20\\\\\\\frac{1}{c} =\frac{1}{20} =0,05\\\\\\Otvet: \ 0,05[/tex]