\frac{AB}{AD}= \frac{BC}{DC}= \frac{AB+BC}{AD+DC}= \frac{9}{AC} \Rightarrow AD= \frac{AB.AC}{9}= \frac{4}{9}AC Ta có \bigtriangleup BAD\infty \bigtriangleup CAB \Rightarrow \frac{AB}{AC}= \frac{AD}{AB}\Rightarrow \frac{4}{AC}= \frac{\frac{4}{9}AC}{4} \Rightarrow AC= 6
(D là chân đường phân giác...