证明:AB=AC,AE为中线,则:∠BAE=∠CAE=(1/2)∠BAC;
又∠CAF=(1/2)∠CAD.
故:∠CAE+∠CAF=(1/2)(∠BAC+∠CAD)=(1/2)*180度=90度.
所以,AE垂直于AF.