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Sum of Angles of a Triangle
Last updated at November 7, 2025 by Teachoo
Class 7 (Ganita Prakash & Old NCERT)
Chapter 7 Class 7 - A tale of three Intersecting Lines (Ganit Prakash)
- Definition
- Constructing Equilateral Triangle
- Constructing a Triangle when its Sides are given
- Figure it out - Page 150, 151
- Are Triangles Possible for any Lengths?
- Figure it out - Page 154
- Visualising the Construction of Circles
- Figure it out - Page 156
- Triangle Inequality & Construction of Circles
- Figure it out - Page 159
- Constructing Triangle when 2 Sides and the Included Angle are given
- Constructing Triangle when 2 Angles and the Included Side are given
- Do triangles always exist?
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Sum of angles of a triangle
- Angle Sum Property
- Exterior Angles
- Constructions Related to Altitudes of Triangles
- Figure it out - Page 170, 171
Transcript
Sum of angles of a triangleConsider a triangle ABC with ∠ B = 50° and ∠ C= 70°. Let us see how we can find ∠ A without construction. Now, we draw a line XY parallel to BC, passing through A Now, we draw a line XY parallel to BC, passing through A Since XY ∥ BC Taking AB as transversal Alternate angles are equal ∴ ∠ XAB = ∠ B ∠ XAB = 50° Since XY ∥ BC Taking AC as transversal Alternate angles are equal ∴ ∠ YAC = ∠ C ∠ YAC = 70° Since XY is a line Applying Linear Pair ∠ XAB + ∠ A + ∠ YAC = 180° 50° + ∠ A + 70° = 180° ∠ A + 120° = 180° ∠ A = 180° – 120° ∠ A = 60°
