Sum of angles in triangle.

**The reasoning** This is one of the most fundamental principles in geometry: **The Angle Sum Property of Triangles**. No matter what type of triangle you're dealing with — whether it's equilateral, isosceles, scalene, right-angled, acute, or obtuse — the three interior angles *always* add up to **180°**. You can prove this by drawing any triangle and extending one side. The exterior angle formed equals the sum of the two opposite interior angles. This relationship only works because the three angles total 180°. Example: In a right-angled triangle, one angle is 90°. If another angle is 50°, the third *must* be 40° because 90° + 50° + 40° = 180°. **Why the wrong options tempt you** - **90°** — That's just *one* angle in a right triangle, not the sum of all three - **270°** — You might confuse this with three right angles multiplied, but they don't all fit in one triangle - **360°** — This is the sum of angles in a *quadrilateral* (four-sided shape), not a triangle **Quick takeaway** Triangle = 180°, Quadrilateral = 360° — the angle sum increases by 180° for each extra side!