1. Lesson 2.1 Measurement of Segments and Angles Objective: Recognize the need for clarity and concision in proofs and understand the concept of perpendicularity

2. From now on, when you write a two-column proof, try to state each reason in a single sentence or less. This chapter contains more definitions and theorems for you to memorize and use.

3. Perpendicularity, right angles and measurements all go together. Perpendicular Lines, Rays and Segments Definition: Lines, rays, or segments that intersect at right angles are perpendicular. What is the symbol for perpendicular?

4. a b F E D M HK J G Let’s Draw some examples of perpendicularity.

5. B A C In the figure at the right, the mark inside the angle ( ) indicates that is a right angle. It is also true that and F E D Do NOT assume perpendicularity from a diagram! In DEF it appears that but we may not assume that they are.

6. In each of the following, name the angles that can be proved to be right angles. M K L J N P O R None S W O T

7. Find the measure of B A C 2 1 Let’s practice Q P R 3 4 S

8. a b 1 2

9. Important … Do NOT assume perpendicularity from a diagram Two perpendicular number lines form a two-dimensional coordinate system, or coordinate plane.

10. The horizontal line is called the x-axis x-axis y-axis The vertical line is called the y-axis Each point is represented by an ordered pair in the form of (x,y) A (3,4) The values of the x and y are called the points coordinates The intersection of the axes is called the origin. Its coordinates are (0,0). Origin

11. Lesson 2.1 Worksheet Homework Summary… Write three things you learned in this lesson.