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NNNLLL54

Ответ:   13 .

Пошаговое объяснение:

AD^2=AC^2+CD^2-2cdot ACcdot CDcdot cosangle C\\\BC=sqrt{AB^2+AC^2}=sqrt{400+225}=sqrt{625}=25\\cosangle C=frac{AC}{BC}=frac{15}{20}=frac{3}{4} \\\AD^2=15^2+4^2-2cdot 15cdot 4cdot frac{3}{5}=225+16-72=169\\AD=sqrt{169}=13

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