2. What is a triangle A plane figure, created by connecting three points that are not in the same line. Three – side polygon whose angles add up to 180°

3. Types of triagles and clasification Classified by Angles 1. Right triangle: when one of the int. angles is a 90° angle 2. Acute triangle: when one of the int. angles is less than 90° 3. Obtuse triangle: when one of the int. Angles is bigger than 90°

4. Classification by sides 1. Equilateral triangle: all 3 sides are the same length, and the angles are the same measure (60°) 2. Isosceles triangle: 2 sides that have equal length, the base angles are the same angle 3. Scalene triangle: has no equal side lengths, or angle measures

5. Parts of a triangle

6. Triangle sum theorem All 3 angles of a triangle add up to 180°