@sonukg4india Mirror drawing with respect to the line BD
Then mirror drawing with respect to the line BC
BJ continues CB
∠FBI=360°–4·75°=60°, BF=BI ⇒ △BFI is equilateral, J is middle of IF
FJ⊥CJ, DA⊥CJ, FJ=BF/2=BC/2=AD ⇒ DAJF is rectangle
DF∥AJ⇒∠DFB=∠FBJ=30°
∠DCB=∠DFB=30°

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