Triangle angle-sum property

👉 The angle-sum property of a triangle states that the sum of the three interior angles of a triangle is always 180°. 🎯

∠A + ∠B + ∠C = 180°


  • This rule applies to all types of triangles — 🔹 acute, 🔸 right, and 🔺 obtuse triangles.
  • Even if the sides or angles change, the total sum of interior angles remains constant (180°)! 🔥

  • Example 1: If ∠A = 60°, ∠B = 70°, find ∠C.
    ✏️ ∠C = 180° – (60° + 70°) = 50°
  • Example 2: If ∠A = 90°, ∠B = 45°, find ∠C.
    ✏️ ∠C = 180° – (90° + 45°) = 45°

👉 The exterior angle of a triangle equals the sum of the two opposite interior angles.

Exterior angle = ∠1 + ∠2

🧠 Example: If one exterior angle = 120°, and one interior opposite angle = 50°,
then the other opposite angle = 120° – 50° = 70°


Triangles are everywhere! 🏠

  • Roofs, signboards, and bridges use triangular shapes because they’re strong and stable — and this property helps engineers calculate angles perfectly! 💪📏

✅ The sum of interior angles of a triangle = 180°
✅ The exterior angle = sum of two opposite interior angles
✅ Applies to all types of triangles
✅ Used in geometry, design, and construction


🌀 “All Triangles Add Up to 180!” 🌀

Learn with an example

Let’s practice!