Definition of a Triangle.

A triangle is a polygon with three straight sides and three vertices (corners). The sum of the interior angles of any triangle is always $180^\circ$.

• Equilateral Triangle: All three sides are equal in length. Consequently, all three interior angles are also equal, each measuring $60^\circ$.
• Isosceles Triangle: Exactly two sides are equal in length. The angles opposite these two equal sides are also equal.
• Scalene Triangle: All three sides have different lengths. Consequently, all three interior angles have different measures.

• *Acute-angled Triangle