The interior angles of a polygon add to a fixed total: triangle $180°$, quadrilateral $360°$, pentagon $540°$.
The rule is $(n-2) \times 180°$ for $n$ sides. In a regular (equal) triangle, each angle is $60°$.
Remember
- Interior angle sum $= (n-2) \times 180°$.
- Triangle 180° · Quadrilateral 360° · Pentagon 540°.