Răspuns:
[tex]AC = \dfrac{AB}{3}[/tex]
[tex]DB = \dfrac{AB}{4}[/tex]
[tex]AC + DB = \dfrac{AB}{3} + \dfrac{AB}{4} = \dfrac{4AB + 3AB}{12} = \dfrac{7AB}{12} \\ [/tex]
[tex]CD = AB - \dfrac{7AB}{12} = \dfrac{12AB - 7AB}{12} = \dfrac{5AB}{12} \\ [/tex]
[tex]\dfrac{5AB}{12} = 30\Rightarrow \bf AB = 72 \ cm[/tex]
R: d) AB = 72 cm
