\[AB = 12\ см;\ \ \ CD = 18\ см.\]
\[Решение.\]
\[1)\ Отношение\ отрезков:\]
\[\frac{\text{AB}}{\text{CD}} = \frac{12}{18} = \frac{2}{3}.\]
\[2)\ AB = 1,2\ дм;\ \ \ CD = 1,8\ дм:\]
\[\frac{\text{AB}}{\text{CD}} = \frac{1,2}{1,8} = \frac{12}{18} = \frac{2}{3}.\]
\[3)\ AB = 120\ мм;CD = 180\ мм:\]
\[\frac{\text{AB}}{\text{CD}} = \frac{120}{180} = \frac{12}{18} = \frac{2}{3}.\]
\[Ответ:\ \ \frac{2}{3};\ нет.\]