\[\boxed{\text{5.42}\text{4}\text{.}}\]
\[1)\ BC = AB - \frac{9}{50} = \frac{17}{50} - \frac{9}{50} =\]
\[= \frac{8}{50}\ м.\ \]
\[2)\ AC = P - (AB + BC) =\]
\[= \frac{17}{20} - \left( \frac{17}{50} + \frac{8}{50} \right) = \frac{17}{20} - \frac{25}{50} =\]
\[= \frac{17}{20} - \frac{1}{2} = \frac{17}{20} - \frac{10}{20} = \frac{7}{20}\ м.\ \]
\[Ответ:AC = \frac{7}{20}\ м.\ \ \]