Please help prove the following proposition: Proposition 3.14 Supplementary angles of congruent angles are congruent.
Is this right? (1) Suppose angle ABC is congruent to angle DEF (given) (2) We have arbitrary points A, C, and G on the sides of angle ABC, and the supplement angle CBG of angle ABC. We can choose points D, F, and H such that AB≅DE,CB≅FE,and BG≅EH. (C-1) (3) Triangle ABC is congruent to triangle DEF (C-6) (4) So AC≅DF and ∡A≅∡D. (def cong triangles) (5) Also AG≅DH (C-3) (6) So triangle ACG is congruent to triangle DFH (C-6 SAS) (7) So CG≅FH and angle G ≅angle H (def cong triangles) (8) So triangle CBD is cong to triangle FEH (C-6 SAS) (9) Then angle CBG is congruent to angle FEH (def cong triangles)