Triangles and Congruence

Classifying Triangles

 A figure formed when three noncollinear points are joined by segments

   Acute Triangle – all acute angles Obtuse Triangle – one obtuse angle Right Triangle – one right angle

   Scalene Triangle – no sides congruent Isosceles Triangle – two sides congruent at least Equilateral Triangle – all sides congruent (also called equiangular)

Angles of a Triangle

 The sum of the measures of the angles of a triangle is 180.

 The acute angles of a right triangle are complementary.

 The measure of each angle of an equiangular triangle is 60.

Geometry in Motion

  When you slide a figure from one position to another without turning it.

Translations are sometimes called slides.

   When you flip a figure over a line. The figures are mirror images of each other. Reflections are sometimes called flips.

  When you turn the figure around a fixed point.

Rotations are sometimes called turns.

  Each point on the original figure is called a pre image.

Its matching point on the corresponding figure is called its image.

 Each point on the pre image can be paired with exactly one point on the image, and each point on the image can be paired with exactly one point on the pre-image.

Congruent Triangles

 If the corresponding parts of two triangles are congruent, then the two triangles are congruent

 The parts of the congruent triangles that “match”

  Δ ABC ≅ Δ FDE The order of the vertices indicates the corresponding parts

  If two triangles are congruent, then the corresponding parts of the two triangles are congruent CPCTC – corresponding parts of congruent triangles are congruent

SSS and SAS

 If three sides of one triangle are congruent to three corresponding sides of another triangle, then the triangles are congruent. (SSS)

 The angle formed by two given sides is called the included angle of the sides

 If two sides and the included angle of one triangle are congruent to the corresponding sides and included angle of another triangle, then the triangles are congruent. (SAS)

ASA and AAS

 If two angles and the included side of one triangle are congruent to the corresponding angles and included side of another triangle, then the triangles are congruent.

 If two angles and a nonincluded side of one triangle are congruent to the corresponding two angles and nonincluded side of another triangle, then the triangles are congruent.