连接OD
∠CAB=30° ∠DOA =60° AD=DC,∠CAB=30° ∠B= 60°
AC=2,BC =2/根号3 =2根号3/3 AB =4根号3/3 AO=2根号3/3
OA =OD ∠DOA =60° AD=AO=2根号3/3