Given: AB = CB, DA = EC
Prove: ∠A = ∠C
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DA + DB = EC + EB
Reflexive
△DBC = △EBA
AE = CD
CPCTC
∠A = ∠C
Reflexive
AB = CB, DA = EC
SAS (Side-Angle-Side)
Substitution
CPCTC
AB = DA + DB and CB = EC + EB
Algebra
SSS (Side-Side-Side)
△DAC = △ECA
Substitution
DB = EB
CB = DA + DB
EC + DB = EC + EB
∠B = ∠B
AC = AC
Given
Segment addition theorem
Substitution