∫ abcd, ABCD, BC⊥AC, ∠ A = 60 in the trapezoid.

∴∠DAC=∠CAB=∠DCA=30

AD = BC

∴DC=AD=BC

∴DA=DC

∫ In Rt△ABC, ∠ ABC = 60, BC⊥AC.

∴AB=2BC

Let ad = a

That is, AD=DC=CB=a and AB=2a.

∫EF is the neutral line.

∴2EF=AB+DC

That is 2 *18 = 2a+a.

Get a= 12.

∴AD=DC=CB= 12,AB=24

∴ trapezoid ABCD perimeter = AD+DC+CB+BA =12+12+12+24 = 60.

That is, the circumference of trapezoidal ABCD is 60cm.