Kẽ: $AK\perp DC;BF\perp DC\\\Rightarrow AK=BF\\AB=KF=4(cm)\\DK=FC=\frac{DC-KF}{2}=\frac{10-4}{2}=\frac{6}{2}=3(cm)$
Có: $DF=DC-FC=10-3=7(cm)\\Xet \Delta ADK(\angle K=90^{\circ}):\\AD^2=AK^2+DK^2(d.ly Pytago)\\\Rightarrow 5^2=3^2+AK^2\\\Rightarrow AK^2=5^2-3^2\\\Rightarrow AK^2=16\\\Rightarrow...