1. triangle – a three sided polygon (straight sides; closed) A B C 3 sides: 3 angles: 3 vertices: A, B, C

2. Classify triangles by sides isosceles triangle – at least two sides are congruent D F E legs: base:

3. Classify triangles by sides scalene triangle – no sides are congruent

4. Classify triangles by sides equilateral triangle – all three sides are congruent

5. Classify triangles by angles acute triangle – all three angles are acute (less than 90º)

6. Classify triangles by angles right triangle – one right angle (exactly 90º) & two acute angles; the acute angles are complementary (they add up to 90°) KM L legs: hypotenuse:

7. Classify triangles by angles obtuse triangle – one obtuse angle (more than 90º) & two acute angles

8. Classify triangles by angles equiangular triangle – all three angles are congruent (60º)

9. altitude or “height” - a segment from a vertex perpendicular to the opposite side; may be inside, outside or on the triangle

10. median – “middle” – segment from a vertex to the midpoint of the opposite side; must be inside the triangle

11. angle bisector – segment from a vertex to the opposite side that bisects the angle