? With what? AD？ Make an equilateral triangle for the edge.

? ∠BAD=∠CAE？

In a triangle? ABD and triangle? Ace clock

AB=AC

AD=AE

∠BAD=∠CAE

So what? Triangle? ABD and triangle? ACE？ suit

So CE=BD=5

∠CDE=∠CDA+∠ADE=？ 30+60? =90?

So in Rt triangle CDE? Medium;

DE=3，CE=5

CD=？ four