triangle – a three sided polygon (straight sides; closed)

A

B C

3 sides:

3 angles:

3 vertices: A, B, C

, ,AB BC CA

, ,A B C

ABC

or

CAB

or

BCA

Classify triangles by sides

isosceles triangle – at least two sides are congruent

D

F

E

legs:

base:

,DF EF

DE

Classify triangles by sides

scalene triangle – no sides are congruent

Classify triangles by sides

equilateral triangle – all three sides are congruent

Classify triangles by angles

acute triangle – all three angles are acute (less than 90º)

Classify triangles by anglesright triangle – one right

angle (exactly 90º) & two acute angles; the acute angles are complementary (they add up to 90°)

KM

L

legs:

hypotenuse:

,MK LK

ML

Classify triangles by angles

obtuse triangle – one obtuse angle (more than 90º) & two acute angles

Classify triangles by angles

equiangular triangle – all three angles are congruent (60º)

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

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

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

bisects the angle