Đáp án: xet tam giac EIC va tam giac FIC
IC chung
ECI=FCI
IEC=IFC
suy ra 2tam giac bang nhau (1)
xet tam giac DBI va tam giac FBI
BI chung
goc FBI bang goc IBD
BDI = IFB
suy ra 2 tam giac bang nhau (2)
xet tam giac BIF va tam giac CIF
IF chung
goc IFC = goc IFB
goc IBF = goc ICF
suy ra 2 tam giac bang nhau (3)
tu (1) (2)(3)
suy ra IE=ID=IF(3 canh t/ung)