Draw the rhombus ABCD, Draw the two diagonals AC and BD and label the intersection as E.
AB ≡ BC ≡ CD ≡ DA : Given. The sides of a rhombus are congruent.
AC ≡ AC ; Reflixivity
ΔACB ≡ ΔACD : SAS
m<EAB = m<ECD : CPCTC
m<EAB = m<ECB : In ΔACB, they are opposite equal sides
m<EAD = m<ECD : In ΔACD, they are opposite equal sides
m<EAB = m<EAD : Both equal to m<ECB
m<ECB = m<ECD : Both equal to m<EAB
BD ≡ BD ; Reflixivity
ΔABD ≡ ΔCBD : SAS
m<EBC = m<EDA : CPCTC
m<EBA = m<EDA : In ΔABD, they are opposite equal sides
m<EBC = m<EDC : In ΔCBD, they are opposite equal sides
m<EBA = m<EBC : Both equal to m<EDA
m<EDC = m<EBC : Both equal to m<EBC
ΔABE ≡ ΔBCE : ASA
ΔABE ≡ ΔCDE : ASA
ΔCDE ≡ ΔDAE : ASA
ΔDAE ≡ ΔABE : ASA
ΔABE ≡ ΔBCE ≡ ΔCDE ≡ ΔDAE : Transitivity
AE ≡ EC : CPCTC ΔABE ≡ ΔCDE
BE ≡ ED : CPCTC ΔABE ≡ ΔCDE
QED