MATH F.2

(1)
AB = AC (given)
ΔABC is an isosceles Δ.
∠ACB = ∠ABC (equal ∠s to equal sides in Δ)

(2)
DF // AC (given)
∠DFB = ∠ACB (corr. ∠s, DF // AC)
∠ACB = ∠ABC (proved in (1))
Hence, ∠DFB = ∠ABC
ΔDBF is an isosceles Δ.
DB = DF (equal sides to equal ∠s in Δ)
But CM = DB (given)
Hence DF = CM

(3)
DF // AC (given)
∠FDE = ∠CME (alt. ∠s, DF // AC)

(4)
DF // AC (已知)
∠DFE = ∠MCE (alt. ∠s, DF // AC)

(5)
Refer to the results in (2), (3) and (4):
ΔDFE ΔMCE (ASA)
Hence, DE = EM (corr. sides, congruent Δs)