Pass through point D as DF⊥BC, and pass through BC at point F.
Pass point d to DE BC, and pass point e to AC.
Know ∞∠CAD = 30...............① ①.
∴DE=？ & ampfrac 12； AD=？ & ampfrac 12； ............. BC (2)
The quadrilateral DECF is a rectangle.
∴de=cf=&； frac 12； ............... BC (3)
That is, point f is the midpoint of side BC,
namely
DF is perpendicular bisector of BC.
∴CD=BD...................④
(After the certificate is completed, I wish you progress every day)