Lesson 3-1

1. Lesson 3-1 Triangle Fundamentals Modified by Lisa Palen

2. Triangle Definition: A triangle is a three-sided polygon. What’s a polygon?

3. Polygons Definition: A closed figure formed by a finite number of coplanar segments so that each segment intersects exactly two others, but only at their endpoints. These figures arenot polygons These figures are polygons

4. Definition of a Polygon A polygon is a closed figure in a plane formed by a finite number of segments that intersect only at their endpoints. From Lesson 3-4

5. Triangles can be classified by: Their sides • Scalene • Isosceles • Equilateral Their angles • Acute • Right • Obtuse • Equiangular

6. Equilateral: A A B C C BC = 3.55 cm B BC = 5.16 cm G H I HI = 3.70 cm Classifying Triangles by Sides Scalene: A triangle in which no sides are congruent. AC = 3.47 cm AB = 3.47 cm AB = 3.02 cm AC = 3.15 cm Isosceles: A triangle in which at least 2 sides are congruent. • A triangle in which all 3 sides are congruent. GI = 3.70 cm GH = 3.70 cm Lesson 3-1: Triangle Fundamentals

7. A triangle in which one angle is.... A ° 44 ° 108 ° 28 C B Classifying Triangles by Angles Obtuse: obtuse. Right: • A triangle in which one angle is... right. Lesson 3-1: Triangle Fundamentals

8. A triangle in which all three angles are.... G ° 76 ° ° 57 47 H I Classifying Triangles by Angles Acute: acute. Equiangular: • A triangle in which all three angles are... congruent. Lesson 3-1: Triangle Fundamentals

9. Classification of Triangles with Flow Charts and Venn Diagrams

10. polygons triangles scalene isosceles equilateral Classification by Sides Polygon Triangle Scalene Isosceles Equilateral

11. polygons triangles right acute equiangular obtuse Classification by Angles Polygon Triangle Right Obtuse Acute Equiangular

12. Naming Triangles We name a triangle using its vertices. For example, we can call this triangle: ∆ABC ∆ACB Review: What is ABC? ∆BAC ∆BCA ∆CAB ∆CBA

13. Parts of Triangles Every triangle has three sides and three angles. For example, ∆ABC has Sides: Angles: CAB ABC ACB

14. Opposite Sides and Angles Opposite Sides: Side opposite of BAC : Side opposite of ABC : Side opposite of ACB : Opposite Angles: Angle opposite of : BAC Angle opposite of : ABC Angle opposite of : ACB Lesson 3-1: Triangle Fundamentals

15. Interior Angle of a Triangle An interior angle of a triangle (or any polygon) is an angle inside the triangle (or polygon), formed by two adjacent sides. For example, ∆ABC has interior angles:  ABC,  BAC,  BCA

16. Exterior Angle An exterior angle of a triangle (or any polygon) is an angle that forms a linear pair with an interior angle. They are the angles outside the polygon formed by extending a side of the triangle (or polygon) into a ray. Exterior Angle Interior Angles A For example, ∆ABC has exterior angle ACD, because ACD forms a linear pair with ACB. D B C

17. Interior and Exterior Angles The remote interior angles of a triangle (or any polygon) are the two interior angles that are “far away from” a given exterior angle. They are the angles that do not form a linear pair with a given exterior angle. Exterior Angle • For example, ∆ABC has exterior angle: • ACD and remote interior angles A and B Remote Interior Angles A D B C

18. Triangle Theorems

19. Triangle Sum Theorem The sum of the measures of the interior angles in a triangle is 180˚. m<A + m<B + m<C = 180 IGO GeoGebra Applet

20. Third Angle Corollary If two angles in one triangle are congruent to two angles in another triangle, then the third angles are congruent.

21. statements reasons Third Angle Corollary Proof Given: The diagram Prove: C  F 1. A  D, B  E 2. mA = mD, mB = mE 3. mA + mB + m C = 180º mD + mE + m F = 180º 4. m C = 180º – m A – mB m F = 180º – m D – mE 5. m C = 180º – m D – mE 6. mC = mF 7. C  F • 1. Given • 2. Definition: congruence • 3. Triangle Sum Theorem • Subtraction Property of Equality • Property: Substitution • Property: Substitution • Definition: congruence QED

22. Corollary Each angle in an equiangular triangle measures 60˚. 60 60 60

23. Corollary There can be at most one right or obtuse angle in a triangle. Example Triangles???

24. Corollary Acute angles in a right triangle are complementary. Example

25. Exterior Angle Theorem The measure of the exterior angle of a triangle is equal to the sum of the measures of the remote interior angles. Remote Interior Angles A Exterior Angle D Example: Find the mA. B C 3x - 22 = x + 80 3x – x = 80 + 22 2x = 102 x = 51 mA = x = 51°

26. Exterior Angle Theorem The measure of an exterior angle of a triangle is equal to the sum of the measures of the remote interior angles. GeoGebra Applet (Theorem 1)

27. Special Segmentsof Triangles

28. Introduction There are four segments associated with triangles: • Medians • Altitudes • Perpendicular Bisectors • Angle Bisectors

29. B C F D E A Median - Special Segment of Triangle Definition: A segment from the vertex of the triangle to the midpoint of the opposite side. Since there are three vertices, there are three medians. In the figure C, E and F are the midpoints of the sides of the triangle. Lesson 3-1: Triangle Fundamentals

30. B B F F D I A D K A Altitude - Special Segment of Triangle The perpendicular segment from a vertex of the triangle to the segment that contains the opposite side. Definition: In a right triangle, two of the altitudes are the legs of the triangle. In an obtuse triangle, two of the altitudes are outside of the triangle. Lesson 3-1: Triangle Fundamentals

31. P M Q O R N L D C Perpendicular Bisector – Special Segment of a triangle A line (or ray or segment) that is perpendicular to a segment at its midpoint. Definition: The perpendicular bisector does not have to start from a vertex! Example: A E A B B In the isosceles ∆POQ, is the perpendicular bisector. In the scalene ∆CDE, is the perpendicular bisector. In the right ∆MLN, is the perpendicular bisector. Lesson 3-1: Triangle Fundamentals