Ta có
$\begin{array}{l} \dfrac{{AC}}{{AD}} = \tan {40^o}\\ \Rightarrow AC = AD\tan {40^o}\\ \dfrac{{AB}}{{AD}} = \tan {25^o} \Rightarrow AB = AD\tan {25^o}\\ \Rightarrow x = AC - AB = AD\left( {\tan {{40}^o} - \tan {{25}^o}} \right)\\ \Rightarrow x = 26\left( {\tan {{40}^o} - \tan {{25}^o}} \right) \approx 9,69\left( m \right) \end{array}$
