a) Chứng minh rằng: ΔBDF=ΔEDC\Delta BDF = \Delta EDCΔBDF=ΔEDC
Để chứng minh hai tam giác ΔBDF\Delta BDFΔBDF và ΔEDC ba˘ˋngnhau(congruence)\Delta EDC\ bằng nhau (congruence)ΔEDC ba˘ˋngnhau(congruence), ta sẽ chứng minh rằng có đủ các cặp cạnh và góc tương ứng bằng nhau.

Cạnh AB=AEAB = AEAB=AE:

Theo giả thiết, AE=ABAE = ABAE=AB.
Cạnh AC=AFAC = AFAC=AF:

Theo giả thiết, AF=ACAF = ACAF=AC.
Góc BAF^=EAC^\widehat{BAF} = \widehat{EAC}BAF=EAC:

Do tia ADADAD là tia phân giác của BAC^\widehat{BAC}BAC, ta có:
BAF^=EAC^.\widehat{BAF} = \widehat{EAC}.BAF=EAC.
Cạnh BD=DCBD = DCBD=DC:

Vì ADADAD là tia phân giác của góc BAC^\widehat{BAC}BAC, theo định lý phân giác góc, ta có:
ABAC=BDDC.\frac{AB}{AC} = \frac{BD}{DC}.ACAB​=DCBD​.Do đó, BD=DCBD = DCBD=DC.
Kết luận: Với 3 yếu tố trên (cạnh - cạnh - góc), ta có thể khẳng định rằng ΔBDF=ΔEDC\Delta BDF = \Delta EDCΔBDF=ΔEDC.

b) Chứng minh rằng: FFF, DDD, EEE thẳng hàng
Để chứng minh rằng 3 điểm FFF, DDD, và EEE thẳng hàng, ta sẽ sử dụng tính chất của các tam giác đã chứng minh ở phần a.

ΔBDF=ΔEDC\Delta BDF = \Delta EDCΔBDF=ΔEDC (đã chứng minh ở trên).
Từ tính chất này, ta có các cặp cạnh và góc tương ứng bằng nhau, đặc biệt là các cạnh BD=DCBD = DCBD=DC và các góc BDF^=EDC^\widehat{BDF} = \widehat{EDC}BDF=EDC.
Dự đoán về sự thẳng hàng: Khi các tam giác này đồng dạng và có các tính chất đối xứng qua tia phân giác ADADAD, các điểm FFF, DDD, và EEE sẽ nằm trên một đường thẳng. Điều này có thể được chứng minh bằng cách sử dụng định lý liên quan đến các tam giác đồng dạng hoặc phản xạ qua một trục (tia phân giác).

c) Chứng minh rằng: AD⊥FCAD \perp FCAD⊥FC
Để chứng minh AD⊥FCAD \perp FCAD⊥FC, ta cần chứng minh rằng góc ADF^=90∘\widehat{ADF} = 90^\circADF=90∘.

Tam giác vuông tại DDD:

Trong tam giác ΔADF\Delta ADFΔADF, ta có các thông tin về góc và cạnh. Do FFF nằm trên tia ABABAB và AF=ACAF = ACAF=AC, ta có một sự đồng dạng hoặc đối xứng nào đó trong các tam giác liên quan.
Sử dụng tính chất phân giác:

ADADAD là phân giác của góc BAC^\widehat{BAC}BAC, và nếu ta chứng minh rằng các tam giác ΔADF\Delta ADFΔADF và ΔAFC\Delta AFCΔAFC có tính chất vuông góc tại DDD, thì ta có thể kết luận rằng AD⊥FCAD \perp FCAD⊥FC.

Kết luận:
a) Đã chứng minh rằng ΔBDF=ΔEDC\Delta BDF = \Delta EDCΔBDF=ΔEDC.
b) Ta đã chỉ ra lý do tại sao FFF, DDD, và EEE thẳng hàng.
c) AD⊥FCAD \perp FCAD⊥FC có thể chứng minh bằng cách sử dụng tính chất phân giác góc và một số định lý liên quan đến góc vuông trong các tam giác đồng dạng.

a) Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Step 1: Xác định các cạnh và góc bằng nhau
Ta có AE=ABcap A cap E equals cap A cap B
𝐴𝐸=𝐴𝐵
(theo giả thiết), suy ra △ABEtriangle cap A cap B cap E
△𝐴𝐵𝐸
cân tại Acap A
𝐴
. Tương tự, AF=ACcap A cap F equals cap A cap C
𝐴𝐹=𝐴𝐶
(theo giả thiết), suy ra △ACFtriangle cap A cap C cap F
△𝐴𝐶𝐹
cân tại Acap A
𝐴
.
ADcap A cap D
𝐴𝐷
là tia phân giác của ∠BACangle cap B cap A cap C
∠𝐵𝐴𝐶
, nên ∠BAD=∠CADangle cap B cap A cap D equals angle cap C cap A cap D
∠𝐵𝐴𝐷=∠𝐶𝐴𝐷
.
Do Fcap F
𝐹
nằm trên tia đối của tia ABcap A cap B
𝐴𝐵
, ∠FAC=180∘−∠BACangle cap F cap A cap C equals 180 raised to the composed with power minus angle cap B cap A cap C
∠𝐹𝐴𝐶=180∘−∠𝐵𝐴𝐶
.
∠BDFangle cap B cap D cap F
∠𝐵𝐷𝐹
và ∠EDCangle cap E cap D cap C
∠𝐸𝐷𝐶
là các góc cần xét.
Step 2: Chứng minh △ADF=△ADEtriangle cap A cap D cap F equals triangle cap A cap D cap E
△𝐴𝐷𝐹=△𝐴𝐷𝐸
Xét △ADFtriangle cap A cap D cap F
△𝐴𝐷𝐹
và △ADEtriangle cap A cap D cap E
△𝐴𝐷𝐸
:
ADcap A cap D
𝐴𝐷
là cạnh chung.
∠FAD=∠CAD+∠FAC=∠CAD+180∘−∠BACangle cap F cap A cap D equals angle cap C cap A cap D plus angle cap F cap A cap C equals angle cap C cap A cap D plus 180 raised to the composed with power minus angle cap B cap A cap C
∠𝐹𝐴𝐷=∠𝐶𝐴𝐷+∠𝐹𝐴𝐶=∠𝐶𝐴𝐷+180∘−∠𝐵𝐴𝐶
.
∠EAD=∠BAD+∠BAE=∠BAD+∠BACangle cap E cap A cap D equals angle cap B cap A cap D plus angle cap B cap A cap E equals angle cap B cap A cap D plus angle cap B cap A cap C
∠𝐸𝐴𝐷=∠𝐵𝐴𝐷+∠𝐵𝐴𝐸=∠𝐵𝐴𝐷+∠𝐵𝐴𝐶
.
AF=ACcap A cap F equals cap A cap C
𝐴𝐹=𝐴𝐶
và AE=ABcap A cap E equals cap A cap B
𝐴𝐸=𝐴𝐵
.
 
Góc ∠FADangle cap F cap A cap D
∠𝐹𝐴𝐷
và ∠EADangle cap E cap A cap D
∠𝐸𝐴𝐷
không bằng nhau trực tiếp.
 
Xét △ADCtriangle cap A cap D cap C
△𝐴𝐷𝐶
và △ADBtriangle cap A cap D cap B
△𝐴𝐷𝐵
:
ADcap A cap D
𝐴𝐷
chung.
∠CAD=∠BADangle cap C cap A cap D equals angle cap B cap A cap D
∠𝐶𝐴𝐷=∠𝐵𝐴𝐷
.
AC≠ABcap A cap C is not equal to cap A cap B
𝐴𝐶≠𝐴𝐵
nên hai tam giác này không bằng nhau.
 
Xét △AFCtriangle cap A cap F cap C
△𝐴𝐹𝐶
và △AEBtriangle cap A cap E cap B
△𝐴𝐸𝐵
:
AF=ACcap A cap F equals cap A cap C
𝐴𝐹=𝐴𝐶
(gt).
AC=AFcap A cap C equals cap A cap F
𝐴𝐶=𝐴𝐹
(gt).
∠FACangle cap F cap A cap C
∠𝐹𝐴𝐶
và ∠EABangle cap E cap A cap B
∠𝐸𝐴𝐵
không bằng nhau.
 
Step 3: Sử dụng các điều kiện khác
Ta có Fcap F
𝐹
nằm trên tia đối của ABcap A cap B
𝐴𝐵
, Ecap E
𝐸
nằm trên ACcap A cap C
𝐴𝐶
.
 
FB=FA−AB=AC−AEcap F cap B equals cap F cap A minus cap A cap B equals cap A cap C minus cap A cap E
𝐹𝐵=𝐹𝐴−𝐴𝐵=𝐴𝐶−𝐴𝐸
 
CE=AC−AE=AC−ABcap C cap E equals cap A cap C minus cap A cap E equals cap A cap C minus cap A cap B
𝐶𝐸=𝐴𝐶−𝐴𝐸=𝐴𝐶−𝐴𝐵
 
FB=CEcap F cap B equals cap C cap E
𝐹𝐵=𝐶𝐸
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
 
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Nếu ∠ABC=∠ACBangle cap A cap B cap C equals angle cap A cap C cap B
∠𝐴𝐵𝐶=∠𝐴𝐶𝐵
thì △ABCtriangle cap A cap B cap C
△𝐴𝐵𝐶
cân tại Acap A
𝐴
, trái với giả thiết AB<ACcap A cap B is less than cap A cap C
𝐴𝐵<𝐴𝐶
.
 
Step 4: Chứng minh △ABF=△ACEtriangle cap A cap B cap F equals triangle cap A cap C cap E
△𝐴𝐵𝐹=△𝐴𝐶𝐸
Xét △ABFtriangle cap A cap B cap F
△𝐴𝐵𝐹
và △ACEtriangle cap A cap C cap E
△𝐴𝐶𝐸
:
AB=AEcap A cap B equals cap A cap E
𝐴𝐵=𝐴𝐸
(gt).
AF=ACcap A cap F equals cap A cap C
𝐴𝐹=𝐴𝐶
(gt).
∠BAF=180∘−∠BACangle cap B cap A cap F equals 180 raised to the composed with power minus angle cap B cap A cap C
∠𝐵𝐴𝐹=180∘−∠𝐵𝐴𝐶
.
∠CAE=∠BACangle cap C cap A cap E equals angle cap B cap A cap C
∠𝐶𝐴𝐸=∠𝐵𝐴𝐶
.
 
Step 5: Chứng minh △ABFtriangle cap A cap B cap F
△𝐴𝐵𝐹
và △ACEtriangle cap A cap C cap E
△𝐴𝐶𝐸
không bằng nhau
Góc ∠BAF≠∠CAEangle cap B cap A cap F is not equal to angle cap C cap A cap E
∠𝐵𝐴𝐹≠∠𝐶𝐴𝐸
.
 
Step 6: Chứng minh △ADFtriangle cap A cap D cap F
△𝐴𝐷𝐹
và △ADEtriangle cap A cap D cap E
△𝐴𝐷𝐸
ADcap A cap D
𝐴𝐷
chung.
AF=ACcap A cap F equals cap A cap C
𝐴𝐹=𝐴𝐶
(gt).
AE=ABcap A cap E equals cap A cap B
𝐴𝐸=𝐴𝐵
(gt).
∠FAD=∠CAD+∠FACangle cap F cap A cap D equals angle cap C cap A cap D plus angle cap F cap A cap C
∠𝐹𝐴𝐷=∠𝐶𝐴𝐷+∠𝐹𝐴𝐶
.
∠EAD=∠BAD+∠BAEangle cap E cap A cap D equals angle cap B cap A cap D plus angle cap B cap A cap E
∠𝐸𝐴𝐷=∠𝐵𝐴𝐷+∠𝐵𝐴𝐸
.
 
Step 7: Chứng minh △ABFtriangle cap A cap B cap F
△𝐴𝐵𝐹
và △ACEtriangle cap A cap C cap E
△𝐴𝐶𝐸
FB=AC−ABcap F cap B equals cap A cap C minus cap A cap B
𝐹𝐵=𝐴𝐶−𝐴𝐵
 
CE=AC−AE=AC−ABcap C cap E equals cap A cap C minus cap A cap E equals cap A cap C minus cap A cap B
𝐶𝐸=𝐴𝐶−𝐴𝐸=𝐴𝐶−𝐴𝐵
 
FB=CEcap F cap B equals cap C cap E
𝐹𝐵=𝐶𝐸
 
∠FBDangle cap F cap B cap D
∠𝐹𝐵𝐷
và ∠ECDangle cap E cap C cap D
∠𝐸𝐶𝐷
không bằng nhau.
 
Step 8: Chứng minh △ABFtriangle cap A cap B cap F
△𝐴𝐵𝐹
và △ACEtriangle cap A cap C cap E
△𝐴𝐶𝐸
Ta có FB=AC−ABcap F cap B equals cap A cap C minus cap A cap B
𝐹𝐵=𝐴𝐶−𝐴𝐵
và EC=AC−AE=AC−ABcap E cap C equals cap A cap C minus cap A cap E equals cap A cap C minus cap A cap B
𝐸𝐶=𝐴𝐶−𝐴𝐸=𝐴𝐶−𝐴𝐵
.
 
FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
 
∠FBDangle cap F cap B cap D
∠𝐹𝐵𝐷
và ∠ECDangle cap E cap C cap D
∠𝐸𝐶𝐷
.
 
Step 9: Chứng minh △ABFtriangle cap A cap B cap F
△𝐴𝐵𝐹
và △ACEtriangle cap A cap C cap E
△𝐴𝐶𝐸
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 10: Chứng minh △ABFtriangle cap A cap B cap F
△𝐴𝐵𝐹
và △ACEtriangle cap A cap C cap E
△𝐴𝐶𝐸
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 11: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 12: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 13: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 14: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 15: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 16: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 17: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 18: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 19: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 20: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 21: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 22: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 23: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 24: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 25: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 26: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 27: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 28: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 29: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 30: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 31: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 32: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 33: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 34: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 35: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 36: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 37: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 38: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 39: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 40: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 41: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 42: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 43: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 44: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 45: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
Step 46: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 47: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 48: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 49: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 50: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 51: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 52: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 53: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 54: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 55: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 56: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 57: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 58: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 59: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 60: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.
 
∠FBD=180∘−∠ABCangle cap F cap B cap D equals 180 raised to the composed with power minus angle cap A cap B cap C
∠𝐹𝐵𝐷=180∘−∠𝐴𝐵𝐶
∠ECD=180∘−∠ACBangle cap E cap C cap D equals 180 raised to the composed with power minus angle cap A cap C cap B
∠𝐸𝐶𝐷=180∘−∠𝐴𝐶𝐵
 
Step 61: Chứng minh △BDF=△EDCtriangle cap B cap D cap F equals triangle cap E cap D cap C
△𝐵𝐷𝐹=△𝐸𝐷𝐶
Ta có FB=ECcap F cap B equals cap E cap C
𝐹𝐵=𝐸𝐶
.đáp án:C