Answer:
1. <1 ~= <4 (Given)
2. <2 is supplementary to <1; <3 is supplementary to <4 (Because <1 & <2, <3 and <4 form linear pairs, they are supplementary by the linear pair postulate.)
3. <2 ~= <3 (By the definition of supplementary angles, m<1 + m<2 = 180 and m<3 + m<4 = 180. By definition of congruent angles, m<1 = m<4, therefore m<2 = m<3 by substitution property of equality.)
4. AB ~= BC (Converse of Isosceles Triangles Theorem)
5. Triangle ABC is isosceles (Definition of Isosceles Triangles)
