Associated Topics || Dr. Math Home || Search Dr. Math

### Parallel Lines and Transversals Proof

```
Date: 09/28/98 at 01:01:19
From: Eric Chan
Subject: Geometric Proofs

I am a freshman at Monta Vista High School in Cupertino, and I am
struggling with Geometric proofs, especially ones that concern
transversals. I find memorizing all the theorems and postulates
difficult, and it is even harder for me to put them to use, for I am
not sure where to put them in. I need help on proving a theorem: "If
two angles are cut by a transversal and the same-side angles are
supplementary, then the lines are parallel."

Thanks,
Eric
```

```
Date: 10/18/98 at 16:47:52
From: Doctor Jaime
Subject: Re: Geometric Proofs

Hello Eric,

If you memorize but you don't understand, it will not be of any use to
you, and you will forget rapidly. You must try to understand what you
are doing.

Let's go to the theorem you give. First draw a diagram this:

Let's suppose angles A and B are supplementary. What can you do with
this information? You may try to get relations between other elements
in the figure, namely angles C and D. It is easy to conclude (do you
know why?) that angles A and D are also supplementary. This means that
angles B and D must be equal. The same reasoning proves that A and C
are equal. You certainly know one theorem that relates equal angles and
parallel lines, don't you? You can now conclude.

In order to give you a better idea how geometric proofs work and how
School/Geometry section of Dr. Math:

http://mathforum.org/dr.math/tocs/geometry.high.html

There is a lot of helpful stuff in there.

If you need more help don't hesitate to write us back.

- Doctor Jaime, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Euclidean/Plane Geometry
High School Geometry
Middle School Geometry
Middle School Two-Dimensional Geometry

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search