For this assignment, I chose to concentrate on the proof found in Theorem 9.10b of the text; “If the slopes of two non-parallel lines in the coordinate plane are equal, then the lines are parallel.” To summarize the proof, Theorem 9.10b is stating that only one line can be drawn that is perpendicular to the given line, line (l). This new line must form four 90 degree angles where it intersects line (1) in order to be parallel. It is further stating that any other line drawn that is perpendicular to line (l) must be parallel to any other line drawn perpendicular to line (l). Questions I would suggest to be asked regarding section 9.3 (the assigned section of this chapter) would be as follows: 1) What does the term “negative reciprocal” mean? 2) How can someone prove a line is parallel or perpendicular to another without using an outside tool (ruler, projector, etc.) I have also attached my real-life examples of the types or lines and angles mentioned in Chapter 9 of the text.