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### Two-Column Proof: Parallel Tangents

```
Date: 03/08/2002 at 00:44:37
From: Andrea
Subject: Two-column proof

I have to prove that tangents to a circle at the endpoints of a
diameter are parallel. I'm not sure how to do a two-column proof, or
I should say I don't understand them. Any advice would help.

Thank you.
```

```
Date: 03/08/2002 at 13:03:24
From: Doctor Peterson
Subject: Re: Two-column proof

Hi, Andrea.

Let's work through this slowly.

What are you supposed to prove?

tangents to a circle at the endpoints of a diameter are parallel

The first thing to do is to extract from this problem a list of the
"givens" (the things you are told are true). Here's the situation:
We've drawn a circle, and then drawn a diameter of it (a chord going
through the center). Then we've drawn the tangent to the circle at
each end of the diameter. I'll make a picture and label the parts:

A                     C
-----------------ooooo+ooooo----------------+
ooooo     |     ooooo
oooo          |          oooo
oo              |              oo
o                |                o
oo                 |                 oo
o                   |                   o
o                    |                    o
o                    |                    o
o                     |                     o
o                    O+                     o
o                     |                     o
o                    |                    o
o                    |                    o
o                   |                   o
oo                 |                 oo
o                |                o
oo              |              oo
oooo          |          oooo
ooooo     |     ooooo
-----------------ooooo+ooooo----------------+
B                     D

So here's what we are given:

Given:
Segment AB is the diameter of a circle with center O
Line AC is tangent to circle O at A
Line BD is tangent to circle O at B

Now, what are we to prove?

To prove:
Lines AC and BD are parallel

Now we have to construct a complete line of reasoning that starts with
the given facts, and ends with the thing we want to prove. How can we
do that?

You can read a number of discussions of how to approach a proof in our
FAQ:

http://mathforum.org/dr.math/faq/faq.proof.html

The basic idea is to look at the facts you have and think of ways you
can use them; and look at the goal and think of ways to approach it.

What do you know about tangents to a circle? Perhaps you have seen a
theorem that says they are perpendicular to the radius. If not, what
theorems do you know about tangents?

What do you know about parallel lines? If you have a transversal (such
as the diameter in this case), what facts about it and the angles it
makes can you use to show that the lines are parallel?

I'll let you go dig in your book for the answers to those questions
(because every book is a little different, so I don't know just what
theorems you can use). Write back and tell me what you've found, and
then we can see how to put them together into a proof.

- Doctor Peterson, The Math Forum
http://mathforum.org/dr.math/
```

```
Date: 03/08/2002 at 20:44:18
From: Andrea
Subject: Re: Two-column proof

GIVEN  -  AC=BD
TO PROVE  -  AC and BD are parallel

STATEMENTS                               REASONS

1.  Point A (outside circle O)           Given
2.  Point B (outside circle O)           Given
3.  Line AC tangent to O                 Given
4.  Line BD tangent to O                 Given
5.  Midpoint of AB is O                  Defining a point
6.  Connect AB through midpoint          Constructing a line
7.  AC is perpendicular to AB            90-degree angle
8.  BD is perpendicular to BA            90-degree angle
9.  AC is parallel to BD                 AC=bd

a           c
oooooooooooooooooooooooooo
oooo
ooo :  ooo
oo   :    oo
oo    :     oo
oo     :      oo
oo      :       oo
oo       :        oo
oo       O        oo
oo       :        oo
oo      :       oo
oo     :      oo
oo    :     oo
oo   :    oo
oo  :   oo
ooo : ooo
ooooo           d
ooooooooooooooooooooooooooo
B

Is there anything else that I would need to do to this? I appreciate
```

```
Date: 03/08/2002 at 22:50:19
From: Doctor Peterson
Subject: Re: Two-column proof

Hi, Andrea.

>GIVEN  -  AC=BD
>TO PROVE  -  AC and BD are parallel

The given is wrong. You don't know anything about the lengths of AC
and BD, if that's what you mean. The "givens" I gave you last time
better reflect what you really know.

>STATEMENTS                               REASONS
>
>1.  Point A (outside circle O)           Given
>2.  Point B (outside circle O)           Given

It would be better to say "_on_ circle O".

>3.  Line AC tangent to O                 Given
>4.  Line BD tangent to O                 Given

I would say "tangent to O _at A_" (or B) to make it clear.

>5.  Midpoint of AB is O                  Defining a point

This isn't really needed; O doesn't show up again. Well, I take that
back: you need to mention somewhere that OA and OB are radii, so
although it is not relevant that O is the midpoint, it is important
that O is on AB. Do you have a theorem that says that a diameter
passes through the center?

>6.  Connect AB through midpoint          Constructing a line

This reads as if you were giving instructions for a construction,
rather than a proof. Constructions are often part of a proof (you may
have to introduce a new line that was not mentioned in the "givens,"
for example; but in this case AB is the diameter you are given, so
there is no need to say this.

>7.  AC is perpendicular to AB            90 degree angle
>8.  BD is perpendicular to BA            90 degree angle

You need a better reason here. No one told you that this is a
90-degree angle, so that can't be the reason it's true.

Do you have a theorem in your text that says that a tangent is
perpendicular to the circle? That's what you need here.

>9.  AC is parallel to BD                 AC=bd

Again, you haven't really given a reason. You need some theorem from
which you can deduce this. AB is a transversal; do the angles BAC and
ABD relate to any theorem you know? Some books prove a theorem that
"two lines perpendicular to the same line are parallel," but without
that, one of the standard transversal theorems will do.

Looking over the proof as a whole, I see that both your "givens" and
your "reasons" are lacking. Each "given" should appear among the
statements somewhere; and each reason should be either "given," a
definition, or a brief statement of a theorem.

Try looking through some sample two-column proofs in our archives to
see what they should look like:

Geometry Proofs
http://mathforum.org/dr.math/problems/victoria.11.07.01.html

http://mathforum.org/dr.math/problems/dement.11.9.99.html

Building Two Column Proofs
http://mathforum.org/dr.math/problems/crystal9.12.98.html

Parallel Lines: Two Column Proof
http://mathforum.org/dr.math/problems/turtle9.9.98.html

Building a Geometric Proof
http://mathforum.org/dr.math/problems/karen.6.03.99.html

Then write back and we can see how you've improved it.

- Doctor Peterson, The Math Forum
http://mathforum.org/dr.math/
```

```
Date: 03/09/2002 at 09:06:48
From: Andrea
Subject: Re: Two-column proof

I think I'm getting better, or at least I hope I am. Here we go.

GIVEN - Segment AB is the diameter of a circle with center O
Line AC is tangent to circle O at A
Line BD is tangent to O at B

TO PROVE - AC and BD are parallel

STATEMENTS                                  REASONS

1.  Point A (on circle O)                    Given
2.  Point B (on circle O)                    Given
3.  Line AC tangent to O at A                Given
4.  Line BD tangent to O at B                Given
5.  Intersect of AB is O                     Definition of Intersects

6.  AC is perpendicualr to AB                If a line in the plane
of a circle is perpendi-
cular to a radius at its
outer endpoint, then the
line is tangent to the
circle (Theorem 9-2)

7. BD is perpendicular to BA                 Theorem 9-2 stated in 6

8. AC is parallel to BD                      Two lines parallel to a
third line are parallel
to each other (Theorem
3-10)

I looked at some of the two-column proofs and I hope I am getting
closer.  These things are not that easy without the help of someone
who understands them.  Thank you for all your help; I would probably
fail this course if it weren't for your site.
```

```
Date: 03/09/2002 at 22:24:21
From: Doctor Peterson
Subject: Re: Two-column proof

Hi, Andrea.

I certainly understand that this can be hard to figure out on your
own; I wish I could just stand there with you and guide you more
interactively. Maybe there is someone you can find - a student who
took geometry last year and wants to improve her knowledge by helping
you? - so you can have someone to share ideas with more quickly.

>GIVEN - Segment AB is the diameter of a circle with center O
>        Line AC is tangent to circle O at A
>        Line BD is tangent to O at B
>
>TO PROVE - AC and BD are parallel
>
>STATEMENTS                                  REASONS
>
>1.  Point A (on circle O)                    Given
>2.  Point B (on circle O)                    Given

Since it isn't the mere fact that these points are on the circle that
you will be using later on, it's probably more useful just to restate
the "given": Segment AB is a diameter of the circle O.

>3.  Line AC tangent to O at A                Given
>4.  Line BD tangent to O at B                Given

You may have to be more careful than I am being about symbolism. Where
I've said "line AC" or "segment AC" there are symbols I can't type,
with a line or segment over the "AC". I'm not sure how best to
indicate "circle O" as opposed to "point O"; sometimes a symbol for a
circle is used, and sometimes the circle is given a separate name,
like "circle c". I don't think I'd just say "tangent to O", though I
have no trouble understanding it. You'll want to check your book's
notation and copy it.

>5.  Intersect of AB is O                     Definition of Intersects

I don't know what you mean by this. There is no such thing as "the
intersect of a segment"; you need two things to intersect. I presume
you mean midpoint, but I don't see how that comes into the rest of the
proof, unless you mean this to suggest that therefore OA is a radius.

>6.  AC is perpendicular to AB                If a line in the plane
>                                             of a circle is perpendi-
>                                             cular to a radius at its
>                                             outer endpoint, then the
>                                             line is tangent to the
>                                             circle (Theorem 9-2)

You don't quite have the right theorem, though I expect that the right
one is near 9-2. Do you see what is wrong? It says "IF it's
perpendicular, THEN it's tangent." You want something that says "If
it's tangent, THEN it's perpendicular," since you know you have a
tangent.

Usually you will just put a brief summary of the theorem ("tangent
written out here for my own sake.

Notice also that this is why you should have a statement that AB is a
diameter; and technically you would probably need another statement
that OA is a radius. I'm not sure whether you would find a theorem
that says half of a diameter is a radius; that might be considered
obvious just from the definitions.

>7. BD is perpendicular to BA                 Theorem 9-2 stated in 6

When two statements have the same reason, just applied to different
parts of the diagram, you can put them into one statement, or say
"same," or do just what you did here.

>8. AC is parallel to BD                      Two lines parallel to a
>                                             third line are parallel
>                                             to each other (Theorem
>                                             3-10)

Did you mean "perpendicular to a third line"? You have no third
parallel line here, so the theorem you have stated isn't what you
need.

Again, I'm commenting on every statement just to give you as much help
as I can, though some comments are trivial and others are important. I
don't want you to be worried about details; the main thing at this
stage is to get a feel for the big idea of a proof. I would be happy
to see a quick summary of the proof: the lines are tangent at the ends
of the diameter, therefore are perpendicular to it by theorem XXX;
since they are perpendicular to the same line, by theorem YYY they are
parallel. The details of the two-column proof sometimes can be
overwhelming and hide what's really doing on; but it is still
worthwhile to be able to write it out that way, in order to see for
yourself that every detail can be covered, and there are no holes in

- Doctor Peterson, The Math Forum
http://mathforum.org/dr.math/
```

```
Date: 03/10/2002 at 20:21:04
From: Andrea
Subject: Re: Two-column proof

Let me start out by saying thank you, I truly believe I'm learning
from all this even though I'm a little slow at it. I have made
corrections to two-column proof and here it is.

GIVEN - Segment AB is the diameter of a circle with center O
Line AC is tangent to circle O at A
Line BD is tangent to O at B

TO PROVE - AC and BD are parallel

STATEMENTS                                     REASONS

1.  Segment AB is a diamter of circle O        Given
2.  Line AC is tangent to O at A               Given
3.  Line BD is tangent to O at B               Given

4.  AC is perpendicular to AB                  Tangent line is perpen-

5.  BD is perpendicular TO BA                  Theorem as stated in
#4 (Theorem 9-1 - If a
line is tangent to a
circle, then the line
is perpendicular to
the point of tangency)

6.  AC is perpendicular to AB                  Two lines perpendicu-
lar to same line are
parallel (Theorem 3-7
In a plane two lines
perpendicular to the
same line are parallel

Thanks so much for all your help, it is greatly appreciated.
```

```
Date: 03/10/2002 at 21:00:49
From: Doctor Peterson
Subject: Re: Two-column proof

Hi, Andrea.

It does take time to get through this, doesn't it? That's probably
because fixing a proof is more like editing an essay than other kinds
of math, and that takes a lot of back-and-forth. I applaud your
persistence, because not many have kept it up this well!

I don't have to go through this line by line, because it's basically
acceptable now. Any further stylistic changes I might suggest wouldn't
benefit your understanding. Assuming this has to be turned in to your
teacher, you may get feedback saying it doesn't fit expectations in
some details - you book may have something more specific to say about
why something perpendicular to the diameter is perpendicular to a
radius as well, and you may have some preferred way to state theorems
in the reasons column - but that's just details. You'll want to learn
to fit the standards, because there's reason for doing so, but I can't
tell you what they are.

- Doctor Peterson, The Math Forum
http://mathforum.org/dr.math/
```

```
Date: 03/10/2002 at 23:32:16
From: Andrea
Subject: Re: Two-column proof

Thank you so much for all your help. It is good to know that you can
find help and learn in the process. It has truly been a learning
experience, thanks to you. Thanks again for all your support and
guidance.
```
Associated Topics:
High School Conic Sections/Circles
High School Euclidean/Plane Geometry
High School Geometry

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