Solution:?
When b is at the midpoint of BC, the quadrilateral CDEF is a parallelogram, ∠ def = 30; In △ADC and △BFC, BF = DC, BC = AC, ∠ B = ∠ ACD ∴△ ADC △ ≌ BFC ∴ AD = FC, ∠DAC =∠BCF = 30∠aed are Bad = 60 ∴ AD bisects the vertical ED ∴ BDE = ∠ DCF = 30 ∴ ED ∴ FC ∴ CDEF is a parallelogram and ∠ DEF = 30.