神马啊搞那么复杂
AB^2=AF^2+FB^2
AC^2=AF^2+FC^2
所以
AB^2-AC^2=FB^2-FC^2=(FB+FC)(FB-FC)
=BC(FB-FC)
=BC[ (BC/2+EF) - (BC/2-EF) ]
= BC * 2EF
=2BC*EF
证毕