Din teorema bisectoarei, avem:
[tex]\dfrac{BD}{DC}=\dfrac{AB}{AC}\Rightarrow \dfrac{DB}{20-DB}=\dfrac{9}{15}[/tex] Adunam numaratorii la numitori, si obtinem:
[tex]\dfrac{DB}{20}=\dfrac{9}{24}\Rightarrow DB=\dfrac{20\cdot9}{24}=7,5\ cm\Rightarrow DC=20-7,5=12,5\ cm[/tex]