18. In quadrilateral ABCD, it is known that △ABC is an equilateral triangle and ∠ ADC = 30? , ad = 3, BD = 5, then the length of the edge CD is _ _ _ _ _ _ _? Is this the right question? Tip: the topic is just a variation of an ordinary topic, and the topic map can be restored to an old topic.

By making an equilateral triangle CDE outside the quadrilateral ABCD with the edge of CD, a right triangle ADE can be constructed, where DE=CD and AE=BD (using the proof of triangle congruence), and then using Pythagorean theorem, CD=4 can be obtained.