①当∠BAC>90º时
BD=√(AB²-AD²)=√(5²-4²)=3
CD=BC-BD=13-3=10
AC=√(CD²+AD²)=√(10²+4²)
=2√29
sinC=AD/AC=4÷2√29=2/29 √29
②当∠BAC<90º时
BD=√(AB²-AD²)=√(5²-4²)=3
CD=CB+BD=13+3=16
AC=√(CD²+AD²)
=√(16²+4²)
=4√17
sinC=AD/AC=4÷(4√17)
=1/17 √17
①当∠BAC>90º时
BD=√(AB²-AD²)=√(5²-4²)=3
CD=BC-BD=13-3=10
AC=√(CD²+AD²)=√(10²+4²)
=2√29
sinC=AD/AC=4÷2√29=2/29 √29
②当∠BAC<90º时
BD=√(AB²-AD²)=√(5²-4²)=3
CD=CB+BD=13+3=16
AC=√(CD²+AD²)
=√(16²+4²)
=4√17
sinC=AD/AC=4÷(4√17)
=1/17 √17