It's actually quite simple. . After passing through point B, point C is perpendicular to AB and AC respectively, and two perpendicular lines intersect at point E, it can be proved that the quadrilateral ABEC is a square, and then it can be proved that △ABD is all equal to △CED and BD=DE=EB, so △BED is an equilateral triangle, so ∠ DBE = 60, so ∠ Abd = 30.