∴5.∠B=∠C (equilateral and equiangular)

∵7.∠ 1=∠B (known)

∴3.∠ 1=∠C (equivalent substitution)

∵2.∠3=∠ 1+∠C (the outer angle of a triangle is equal to the sum of two non-adjacent inner angles)

∴8.∠3=2∠ 1 (equivalent substitution)

6.2 ∫ 1∠2 =∠ 1, that is ∠2=2∠ 1 (known).

∴9.∠2=∠3 (equivalent substitution)

∴4.AB=BD, that is, △ABD is an isosceles triangle (the definition of isosceles triangle).