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Class 7 Maths- Ch 6 – Triangle and its properties Angle Sum Property The total measure of the three angles of a triangle is 180°. Given: -∠1,∠2,∠3 are angles of  ∆ABC ∠4 is the exterior angle when BC is extended to D Justification ∠1+∠2=∠4(by exterior angle property) ∠1+∠2+∠3=∠4+∠3 (adding ∠3 on both sides) ∠4 and ∠3 form a linear pair so it is 180° Hence ∠1+∠2+∠3=180° Watch and learn more about angle sum property –  Class 7 Maths . Two Special Triangles: Equilateral and Isosceles Equilateral Triangle: –  Triangles in which all the three sides are equal are called an equilateral triangle. Two Important points:- all sides have the same length each angle has measure 60° Isosceles triangle: –  Triangles in which two sides are of equal lengths is called an isosceles triangle. Two Important points:- two sides have the same length base angles opposite to the equal sides are equal Read full article click here.... Class 7 Maths- Ch 6 – Triangle an