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Cass 7 Maths online solutions-Triangle and its properties Chapter 6 – Triangle and its properties  Introduction Exterior angle of a triangle and its properties An exterior angle of a triangle can be formed by extending the sides of a triangle at its vertex. A triangle can have three exterior angles. An exterior angle of a triangle is equal to the sum of its interior opposite angles. Proof:- Given  – Consider ∆ABC ∠ACD is an exterior angle To Show:- M ∠ACD=m ∠A +m ∠B Through C draw CE parallel to BA ∠1= ∠X (BA is parallel to CE and AC is a transversal so the alternate angle should be equal) ∠2= ∠y (BA is parallel to CE and AD is a transversal so the corresponding angle should be equal) ∠1+ ∠2= ∠x+ ∠y As we know ∠x+ ∠y=m ∠ACD So, ∠1+ ∠2= ∠ACD Hence proved The above relation proved between an exterior angle and its two interior opposite angles are referred as the exterior angle property of a triangle. For examples and practice questions click o