1. Lesson 4
Triangle Basics

3. Definition
A triangle is a three-sided figure formed by joining three line segments together at their endpoints.
A triangle has three sides.
A triangle has three vertices (plural of vertex).
A triangle has three angles. 3 2 1

4. Naming a Triangle
Consider the triangle shown whose vertices are the points A, B, and C.
We name this triangle by writing a triangle symbol followed by the names of the three vertices (in any order). C
Name A B

5. The Angles of a Triangle
The sum of the measures of the three angles of any triangle is
Let’s see why this is true.
Given a triangle, draw a line through one of its vertices parallel to the opposite side.
Note that because these angles form a straight angle.
Also notice that angles 1 and 4 have the same measure because they are alternate interior angles and the same goes for angles 2 and 5.
So, replacing angle 1 for angle 4 and angle 2 for angle 5 gives 4 5 3 1 2

6. Example In
What is

7. Example In the figure, is a right angle and bisects If then what is A25 50 40 ? 65 90

8. Angles of a Right Triangle
Suppose is a right triangle with a right angle at C.
Then angles A and B are complementary.
The reason for this is that B A C

9. Exterior Angles
An exterior angle of a triangle is an angle, such as angle 1 in the figure, that is formed by a side of the triangle and an extension of a side.
Note that the measure of the exterior angle 1 is the sum of the measures of the two remote interior angles 3 and 4. To see why this is true, note that 4 1 2 3

10. Classifying Triangles by Angles
An acute triangle is a triangle with three acute angles.
A right triangle is a triangle with one right angle.
An obtuse triangle is a triangle with one obtuse angle.
acute triangle
right triangle
obtuse triangle

11. Right Triangles
In a right triangle, we often mark the right angle as in the figure.
The side opposite the right angle is called the hypotenuse.
The other two sides are called the legs. B C A
hypotenuse
leg
leg

12. Classifying Triangles by Sides
A triangle with three congruent sides is called equilateral.
A triangle with two congruent sides is called isosceles.
A triangle with no congruent sides is called scalene.
equilateral
isosceles
scalene

13. Angles and Sides If two sides of a triangle are congruent…
then the two angles opposite them are congruent.
If two angles of a triangle are congruent…
then the two sides opposite them are congruent.

14. Equilateral Triangles
Since all three sides of an equilateral triangle are congruent, all three angles must be congruent too.
If we let represent the measure of each angle, then

15. Isosceles Triangles Suppose is isosceles where
Then, A is called the vertex of the isosceles triangle, and is called the base.
The congruent angles B and C are called the base angles and angle A is called the vertex angle. B A C

16. Example is isosceles with base If is twice then what is
Let denote the measure of
Then A x 2x 2x B C

17. Example In the figure, and Find Since is isosceles,
the base angles are congruent. So, A 25
130 D 25 50
110 B 20 C

18. Inequalities in a Triangle
In any triangle, if one angle is smaller than another, then the side opposite the smaller angle is shorter than the side opposite the larger angle.
Also, in any triangle, if one side is shorter than another, then the angle opposite the shorter side is smaller than the angle opposite the longer side.

19. Example Rank the sides of the triangle below from smallest to largest.
First note that
So, C A B

20. Medians
A median in a triangle is a line segment drawn from a vertex to the midpoint of the opposite side.
An amazing fact about the three medians in a triangle is that they
all intersect in a common
point. We call this
point the centroid
of the triangle.

21. Another fact about medians is that the distance along a median from the vertex to the centroid is twice the distance from the centroid to the midpoint. 2x x

22. Example In the medians are drawn, and the centroid is point G. Suppose
Find A N
4.5 G C P 4 7 M B

23. Midlines
A midline in a triangle is a line segment connecting the midpoints of two sides.
There are two important facts about a midline to remember:
midline x 2x

24. Example In D and E are the midpoints of respectively.
If and then find and A C D E B

25. The Pythagorean Theorem
Suppose is a right triangle with right angle at C.
The Pythagorean Theorem states that
Here’s another way to state the theorem: label the lengths of the sides as shown. Then B C A a b c

26. In words, the Pythagorean Theorem states that the sum of the squares of the lengths of the legs equals the square of the length of the hypotenuse, or:
leg
hypotenuse

27. Example
Suppose is a right triangle with right angle at C. B C A

28. Triangles
A triangle is a triangle whose angles measure
It is a right triangle and it is isosceles.
If the legs measure then the hypotenuse measures
This ratio of the sides is memorized, and if one side of a triangle is known, then the other two can be obtained from this memorized ratio. 45

29. Example In is a right angle and If then find
First notice that too since the angles must add up to
Then this is a triangle and so: B 6 ? 45 A C

30. Triangles
A triangle is one in which the angles measure
The ratio of the sides is always as given in the figure, which means:
The side opposite the angle is half the length of the hypotenuse.
The side opposite the angle is times the side opposite the angle. A B C

31. Example In
If find
First note that, since the three angles must add up to
So this is a triangle. A C B

32. The Converse of the Pythagorean Theorem
Suppose is any triangle where
Then this triangle is a right triangle with a right angle at C.
In other words, if the sides of a triangle measure a, b, and c, and
then the triangle is a right triangle where the hypotenuse measures c.

33. Example
Show that the triangle in the figure with side measures as shown is a right triangle. 7 24 25