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Geometry Session 4, Part B: Parallel Lines
Session 4 Part A Part B Part C Part D Homework
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Session 4 Materials:

Session 4, Part B:
Parallel Lines

In This Part: Properties of Angles | Reasoning About Properties of Angles

In the previous subpart, you gathered evidence of some properties of angles. But evidence alone doesn't explain why something is true, or even mean that it is true. Mathematics requires a reasoned argument that is general, not about a specific set of lines.

When two lines intersect, the vertical angles (angles opposite each other) have the same measure.


Problem B5


In this problem, you will look at an explanation for why vertical angles have the same measure.


mangle1 + mangle2 = 180°. Why?


Also mangle3 + mangle2 = 180°. Why?


So mangle1 must equal mangle3. Why?


What other pair of angles is equal in measure? Why?


When two parallel lines both intersect a third line, corresponding angles (angles in the same relative positions, like angles 1 and 7 or angles 3 and 5 in the picture below) have the same measure.

One way to understand this is to imagine sliding a copy of the picture above along line j until line k sits on top of line l. Note 3

Now angle1 sits exactly where angle7 used to be, angle3 sits exactly where angle5 used to be, and so on.

To prove that corresponding angles are congruent, we could add another line segment, , parallel to line j. By doing so we have created a parallelogram, and thus we know that the adjacent angles of a parallelogram (in this case 2 and 7) equal 180°.

So, to prove that 1 and 7 are congruent, we write the following:

1 + 2 = 180° (because they form a straight line)
2 + 7 = 180° (because they are adjacent angles of a parallelogram)

It follows that 1 + 2 = 2 + 7 = 180°.

And thus, 1 = 7


Alternate interior angles (angles on opposite sides of the transversal, and between the parallel lines, like angle7 and angle3 or angle2 and angle8), also have the same measure.


Problem B6


Create an argument to explain why the above statement about alternate interior angles would be true. You may want to use the facts about corresponding angles and vertical angles, or you may come up with another explanation.

Take it Further

Problem B7


Create any quadrilateral with Geometer's Sketchpad. Construct the midpoints of all four sides of the quadrilateral, and then connect them in order.



Describe the quadrilateral in the center.


How could you test to see if you are right about the kind of quadrilateral you have in the center? What measurements could you make?


Move around your original quadrilateral, changing its shape. What changes on the inside quadrilateral? What stays the same?


How can you change your original quadrilateral to make the inside quadrilateral a square?



Problem B5 adapted from IMPACT Mathematics, Course 1, developed by Educational Development Center, Inc. p. 473. © 2000 Glencoe/McGraw-Hill. Used with permission. www.glencoe.com/sec/math

Problem B6 developed by Educational Development Center, Inc. © 2000 Glencoe/McGraw-Hill. Used with permission. www.glencoe.com/sec/math

Next > Part C: Circles

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