Plane Geometry - Bruce E. Shapiro

Section 24Exterior AnglesDefinition 24.1 Let △ABC be a triangle. The angles ∠ABC, ∠BCA, ∠CABare called interior angles of △ABC.Definition 24.2 An exterior angle of a triangle is any angle that formsa linear pair with an interior angle of the same triangle.Definition 24.3 If an exterior angle forms a linear pair with the interiorangle at one vertex, the other two interior angles are called remote interiorangles.Figure 24.1: Angle α, β, and γ are interior angles of this triangle. ɛ andδ are exterior angles because the each form a linear pair with the interiorangle α. Angles β and γ are remote interior angles of ɛ and δ. By theexterior angle theorem, ɛ > β, ɛ > γ, δ > β, δ > γ.115