连接AC，∵∠DBA和∠DCA都为AD所对的圆周角，∴∠DBA=∠DCA，∵AB为⊙O的直径，∴∠BCA=90°，∴∠CBA+∠CAB=90°，∵∠CAB=∠E+∠DCA，∴∠CBD+∠DBA+∠E+∠DBA=90°，∵∠E=20°，∠DBC=50°，∴∠DBA=10°，∴∠C...