a) Theo tinh chat phan giac ta co :
AD/CD = AB/BC (1)
BE/AE = BC/AC (2)
Lay (1) x (2) : (AD/CD)(BE/AE) = AB/AC < 1 ( vi AB < AC)
<=> AD/CD < AE/BE
<=> AK/BK < AE/BE ( do KD//BC => AD/CD = AK/BK)
<=> (AB - BK)/BK < (AB - BE)/BE
<=> AB/BK - 1 < AB/BE - 1
<=> AB/BK < AB/BE <=> 1/BK < 1/BE
<=> BK > BE => E nam giua B va K (dpcm)
b) Theo ket qua cau a) => ^EDB < ^KDB = ^CBD ( so le trong vi KD//BC) = ^EBD ( vi BB la phan giac) => BE < ED (3) ( trong tg doi dien voi goc be hon la canh be hon)
Tuong tu nhu the neu ke EF//BC ( F thuoc AC ) => F nam giua C va D do do ta cung co ^DCE = ^BCE = ^FEC < ^DEC => ED < CD (4)
Tu (3) va (4) co : BE < ED < CD ( dpcm)