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Math 409, Spring 2013 Axioms, definitions and theorems for plane geometry Last update: February 18, 2013
The building blocks for a coherent mathematical system come in several kinds:
- Undefined terms. These are typically extremely simple and basic objects (like “point” and “line”), so simple that they resist being described in terms of simpler objects. Every system has to have some undefined terms—you’ve got to start somewhere. (But in general, the fewer the better.)
- Postulates/Axioms. These are basic facts about undefined terms. The simpler and more funda- mental they are, the better. For example, “every pair of points determines a line”, or “if x = y, then y = x.”
- Definitions. We can define new terms using things that we already know.
- Theorems. These are the statements that make mathematics what it is—they are facts that we prove using axioms, definitions, and theorems that we’ve proved earlier. (Propositions, Lemmas, Corollaries are all species of theorems.)
1 Undefined terms
We will start with the following undefined terms:^1
- Point
- Line (= infinite straight line)
- Angle (determined by three points A, B, C, with A 6 = C and B 6 = C)
- Distance between two points
- Measure of an angle
We’ll write AB for the distance between two points A and B, and we’ll write m∠ABC for the measure of angle ∠ABC.
We’ll be careful to distinguish between an angle (which is a thing) and its measure (which is a number). So, for instance, the two statements “∠ABC = ∠XY Z” and “m∠ABC = m∠XY Z” don’t mean the same thing—the first says these two angles are actually the same angle, while the second just says that they have the same measure.
2 Some definitions
When writing definitions, it is good practice to emphasize the word you are defining—that makes it easier on the reader (and for that matter the writer as well).
(^1) Euclid included definitions of these terms in the Elements, but to a modern reader, his definitions are really intuitive explanations rather than precise mathematical definitions. For example, he defined a point as “that which has no part”, a line as “length without breadth”, and an angle as “the inclination to one another of two lines in a plane which meet one another and do not lie in a straight line.” (The intuition here is that an angle records the difference between the directions of two lines meeting in a point.)
Definition 1. A collection of three or more points is collinear if there is some line containing all those points.
Definition 2. Two lines are parallel if they never meet.
Definition 3. When two lines meet in such a way that the adjacent angles are equal, the equal angles are called right angles, and the lines are called perpendicular to each other.
Definition 4. A circle is the set of all points equally distant from a given point. That point is called the center of the circle.
What about the term “line segment”? We all know what that is—it’s the portion of a line between two points. But what does “between” mean?
With a little thought, we can define “between” using two concepts we already have: the undefined term distance and the definition of collinear. Once we’ve done that, we can define what a line segment is. It’s important to get these two definitions in the proper order.
Definition 5. Given three distinct collinear points A, B, C, we say that B is between A and C if AC > AB and AC > BC.
Definition 6. The line segment AB between two points A and B consists of A and B themselves, together with the set of all points between them.
Warning: We often talk about “parallel line segments”, but we have to be careful about what “parallel” means in this contexts. Clearly two line segments can be non-parallel even if they never meet, like this: — |. So if we say that “two line segments are parallel”, we really mean that the lines they lie on are parallel (see Definition 2).
3 Axioms
Axiom 1. If A, B are distinct points, then there is exactly one line containing both A and B, which we denote
AB (or
BA).
This is Euclid’s first axiom. Notice that it includes a definition of notation.
The next group of axioms concern distance.
Axiom 2. AB = BA.
Axiom 3. AB = 0 iff A = B.
The word “iff” is mathematician’s jargon for “if and only if”. That is, the axiom says that two different things are true. First, if A = B, then AB = 0. Second, if AB = 0, then A = B. Logically, these are two separate statements.
Axiom 4. If point C is between points A and B, then AC + BC = AB.
Axiom 5. (The triangle inequality) If C is not between A and B, then AC + BC > AB.
Now, some axioms about angle measure.
Axiom 6. (a.) m(∠BAC) = 0◦^ iff B, A, C are collinear and A is not between B and C.
(b.) m(∠BAC) = 180◦^ iff B, A, C are collinear and A is between B and C.
Axiom 10 (which is called Playfair’s Axiom) and Axiom 11 distinguish Euclidean geometry from other geometries, such as spherical geometry (which we’ve talked a little about) and hyperbolic geometry (which we’ll see eventually). Both axioms are intuitively correct, but it took mathematicians a long time to realize that it was possible to do geometry without them.
Now for two axioms that connect number and geometry:
Axiom 12. For any positive whole number n, and distinct points A, B, there is some C between A, B such that n · AC = AB.
Axiom 13. For any positive whole number n and angle ∠ABC, there is a point D between A and C such that n · m(∠ABD) = m(∠ABC).
4 Some theorems
Now that we have a bunch of axioms and definitions in place, we can start using them to prove theorems. (We reserve the right to add more axioms and definitions later if we need to.) Many of these theorems may seem obvious, but that’s the point: even seemingly obvious statements need to be proved.
The first theorem was actually one of Euclid’s original five postulates (= axioms). In our axiom system, which is not the same as Euclid’s, we don’t need to make it an axiom—we can prove it from the axioms and definitions above.
Theorem 1. All right angles have the same measure, namely 90 ◦.
Proof. Suppose that ∠ABX is a right angle. By Definition 3, this means that the segments AB and BX can be extended to perpendicular lines
AB and
BX. Let C be a point on
AB such that B is between A and C. Now, by Definition 3 of “perpendicular”, we know that
m∠ABX = m∠XBC
and by Axiom 8, we know that m∠ABX + m∠XBC = 180◦.
Substituting the first equation into the second, we find that 2m∠ABC = 180◦, so m∠ABC = 90◦.
Notice that this proof says explicitly when it using an axiom or definition. This is an important habit to acquire when writing proofs—it’s just like citing your sources.
By the way, the little box at the end is a sign that the proof is complete. (The old practice was to use the abbreviation “Q.E.D.”, a Latin acronym meaning “... which was to be proven”.)
The next several theorems say that certain things are unique. every line segment has exactly one midpoint, every angle has exactly one bisector, and every line has exactly one perpendicular through a point on it.
Definition 7. A midpoint of a line segment AB is a point C on AB such that AC = BC and 2 · AC = AB.
Theorem 2. Every line segment AB has exactly one midpoint.
Proof. First, we show that AB has at least one midpoint. By Axiom 12, we can find a point C between A and B (that is, on AB) such that 2 · AC = AB. (1)
So we just need to show that AC = BC. By Axiom 4, we also know that AC + BC = AB so substituting for AB in equation (1) (which we can do by Axiom 9) gives us 2 · AC = AC + BC, and subtracting AC from both sides gives AC = BC,
The second part of the proof is to show that AB has only one midpoint. To do this, suppose that C and D are both midpoints—the goal is then to show that in fact C = D. By Axiom 3, we can do this by showing that CD = 0. We don’t know anything about CD directly; what we do know is that since C and D are both midpoints of AB,
AC = CB = AD = DB =
AB
Now C is either between A and D, or between B and D If C is between A and D, then Axiom 4 says that AC + CD = AD. But since AC = AD (by equation (2)), this means that CD = 0.
If C is between B and D, then Axiom 4 says that BC + CD = BD. But since BC = BD (by equation (2)), this means that CD = 0.
In either case, Axiom 3 tells us that C = D.
Notice that the two cases were essentially the same, In a written proof, you might see the author say something like, “Without loss of generality, we’ll just consider the first case”—that’s what this means.
Definition 8. A bisector of an angle ∠BAC is a line
AD such that D is between B and C and m∠BAD = m∠DAC = 12 m∠BAC.
Theorem 3. Every angle ∠BAC has exactly one bisector.
Proof. By Axiom 13, ∠BAC has at least one bisector. We have to show that it has only one.
Suppose that
AD and
AE are both bisectors. Then D and E are points on BC. So D is either between B and E, or between E and C. Without loss of generality, we’ll consider the first case. By Axiom 8,
m∠BAD + m∠DAE = m∠BAE.
On the other hand, since
AD and
AE are both bisectors, we know from Definition 8 that
m∠BAD = m∠BAE =
m∠BAC.
Substituting this into the previous equation gives m∠BAD + m∠DAE = m∠BAD, which implies that m∠DAE = 0. By Axiom 6, the points A, D, E are collinear — but that means that
AD =
AE, so both bisectors are the same.
As an immediate corollary, we get the following uniqueness result:
α ’ α ’’
m’
m
A
n
A’
β
α
Let’s prove the “alternate interior angles” case. We are given that α = α′, and we want to prove that m and m′^ are parallel.
Suppose that m and m′^ meet at a point Z. Then we have a triangle ∆AZZ′. By Theorem 6,
m∠AZZ′^ + m∠AZ′Z + m∠ZAZ′^ = α + β + m∠ZAZ′^ = 180◦.
On the other hand, α + β = 180◦^ by Axiom 8. Therefore, m∠ZAZ′^ = 0, which says that Z, A, Z′^ are collinear—but they’re not. This is a contradiction, and we conclude that m and m′^ do not meet.
As for the “corresponding angles” case, alternate interior angles (such as α and α′) are equal if and only if corresponding angles (such as α and α′′) are equal. This is because of the Vertical Angle Theorem (which says that α′^ = α′′).
5 Congruence and similarity
Definition 9. Two things are congruent iff one of them can be moved rigidly so that it coincides with the other. In particular, if one of them consists of line segments then so does the other, and corresponding sides have the same measure. We write F ∼= G to mean that F and G are congruent.
Definition 10. Two things are similar iff one of them is proportional to the other. In particular, if one of them consists of line segments then so does the other, and corresponding sides have proportional measures. We write F ∼ G to mean that F and G are similar.
Notice that “congruent” is a stronger relationship than “similar”. If two things are congruent, then they are necessarily similar, but two similar things don’t have to be congruent.
Axiom 14. (SSS) Two triangles are congruent iff their corresponding sides are equal. That is, if ∆ABC and ∆A′B′C′^ are two triangles such that AB = A′B′, AC = A′C′, and BC = B′C′, then ∆ABC ∼= ∆A′B′C′.
We already know from Definition 9 that if the triangles are congruent, then corresponding sides are equal. What is new in Axiom 14 is the reverse implication: if corresponding sides are equal, then the triangles are congruent.
Axiom 15. (AAA) Two triangles are similar iff their corresponding angles are equal. That is, if m∠BAC = m∠B′A′C′, m∠ABC = m∠A′B′C′, and m∠BCA = m∠B′C′A′, then ∆ABC ∼ ∆A′B′C′.
The abbreviations SSS and AAA are short for “Side-Side-Side” and “Angle-Angle-Angle”. It is natural to ask about other criteria for congruence of triangles.
Theorem 8. (ASA) Two triangles are congruent iff two pairs of corresponding angles, and the sides between them, are equal. That is,
∆ABC ∼= ∆A′B′C′^ iff m∠BAC = m∠B′A′C′, m∠ABC = m∠A′B′C′, and AB = A′B′.
Proof. To prove a theorem with an “iff” in its statement, we need to prove that both implications hold. In this case, it is easy to prove that if the triangles are congruent, then the three equalities actually hold — this is an immediate consequence of Definition 9.
So suppose we have two triangles that satisfy the three equalities. Draw then so that the points A, B, A′, B′ lie along the same line n, in that order. Also, let
α = m∠BAC, β = m∠ABC α′^ = m∠B′A′C′, β′^ = m∠A′B′C′.
B’
A
A’
B
C
C’
α
β
α
β
’
n
By Theorem 7, we know that
AC and
A′C′^ are parallel (because they both intersect n, and the corresponding
angles α, α′^ are equal). By similar logic, we know that
BC and
B′C′^ are parallel.
Now, slide ∆A′B′C′^ along n so that the segments A′B′^ and AB coincide with each other, i.e., A = A′^ and B = B′. (We know we can do this because AB = A′B′^ by hypothesis.) Theorem 7 still applies, but here
it says that
AC and
A′C′^ are actually the same line (they can’t be parallel because they both contain the point A = A′, so according to the theorem the only other possibility is that they weren’t distinct lines to
begin with). Similarly,
BC and
B′C′^ coincide.
Let ` =
AC =
A′C′^ and m =
BC =
B′C′. The lines ` and m intersect in a unique point, by Theorem 5. But that unique point must be both C and C′^ — so we conclude that C = C′, and the proof is done.
An equivalent way of stating the ASA theorem is as follows:
A triangle is determined, up to congruence, by one side and the two angles adjacent to it.
Third (and slickest) proof. Observe that AB = AC, AC = AB, and BC = CB. Therefore ∆ABC ∼= ∆BAC by SSS. In particular, ∠ABC ∼= ∠ACB. (The big idea here is that the triangle is congruent to a reflected copy of itself.)
B D C B E C B C
A A A
Proof 1 Proof 2 Proof 3
The points D and E are actually the same point, but we can’t assume that from the start — so it is a logical mistake to say something like, “Let be the bisector of ∠BAC, and let meet BC at its midpoint”. To put it another way, you can’t assume that you can construct ` in a way that meets both those specifications. On the other hand, we can now prove that D and E coincide.
Finally, two more very useful equalities about angles, triangles and semicircles.
Theorem 11. Suppose that AB is a diameter of a circle centered at O, and that C is a point on the circle. Then m∠ACB = 90◦^ (3a)
and m∠BOC = 2m∠BAC. (3b)
Proof. The segments OA, OB, OC are all radii of the circle, so the triangles ∆OAC and ∆OBC are isosceles. Therefore, by Theorem 10,
m∠OAC = m∠OCA and m∠OCB = m∠OBC.
Let α = m∠OAC = m∠OCA and β = m∠OCB = m∠OBC. By Axiom 8 and Theorem 6,
2 α + 2β = m∠OAC + m∠OCA + m∠OCB + m∠OBC = m∠BAC + m∠ACB + m∠ABC = 180 ◦
from which it follows that α + β = 90◦, proving (3a).
θ
C
α
α β
β
A O B
Now, let θ = m∠BOC. Then θ + 2β = 180◦
and we already know that 2 α + 2β = 180◦
and combining these two equations yields θ = 2α
which proves (3b).
6 The Pythagorean Theorem
Theorem 12. In a right triangle with legs of lengths a and b and hypotenuse of length c,
a^2 + b^2 = c^2.
Proof. Suppose we are given such a right triangle ∆AW X, where W X is the hypotenuse (see figure, left, below ). Construct a square W XY Z using the hypotenuse as one side (see figure, right, below ).
c
a c
b
c
c
Y
Z
W
A (^) X
a c
b
W
A (^) X
7 Definitions and theorems about quadrilaterals
Definition 11. A quadrilateral Q = ABCD consists of four points A, B, C, D and the line segments
AB, BC, CD, DA. The diagonals of Q are the line segments AC and BD. The quadrilateral is called convex if the diagonals cross each other, but AB does not meet CD and BC does not meet DA. All quadrilaterals we’ll consider will be convex.
Theorem 13. [EG 23] The angles of every quadrilateral add up to 360 ◦.
Proof. Draw the quadrilateral ABCD and the diagonal AC. Label the angles as shown.
α B
C
D
A
Then,
m∠ABC + m∠BCD + m∠CDA + m∠DAB = β + (γ + θ) + δ + (ε + α) (by Axiom 8) = (α + β + γ) + (θ + δ + ε) = 180◦^ + 180◦^ = 360◦^ (by Theorem 6).
Definition 12. A quadrilateral Q = ABCD is a parallelogram if
AB is parallel to
CD and
BC is parallel to ←→ AD. It is a rectangle if ∠ABC, ∠BCD, ∠CDA, ∠DAB are all right angles. It is a rhombus if AB = BC = CD = DA. It is a square if it is both a rectangle and a rhombus.
The next several theorems are about parallelograms.
Theorem 14. [EG 27] In a parallelogram P QRS, opposite sides and opposite angles are equal. That is, if ←→ P Q is parallel to
RS and
P S is parallel to
QR, then
P Q = RS and P S = RQ (6a)
and ∠P QR ∼= ∠RSP and ∠QRS ∼= ∠SP Q. (6b)
Proof. Draw the diagonal P R. By Axiom 11, ∠SRP ∼= ∠RP Q and ∠SP R ∼= ∠QRP. Also, P R = RP (Axiom 2), so by ASA (Theorem 8), we have congruent triangles: ∆P RS ∼= ∆RP Q. In particular, RS = P Q and P S = RQ = QR, proving (6a). Also, ∠RSP ∼= ∠P QR, which is one of the assertions of (6b), and we can obtain the other equality by constructing the diagonal QS and arguing similarly.
P
S R
Q
X
The next theorem is a well-known fact about parallelograms, but does take a little effort to prove. Once we know it, it will be very useful in the following two results about special kinds of parallelograms (that is, rhombi and rectangles).
Theorem 15. The diagonals of every parallelogram bisect each other. That is, if P QRS is any parallelogram, and X = P R ∩ QS is the point where its diagonals meet, then P X = RX and QX = SX.
Proof. By Axiom 11 and the fact that P, X, R are collinear, we have
∠XRS = ∠P RS ∼= ∠RP Q = ∠XP Q
and similarly by Axiom 11 and the fact that Q, X, S are collinear,
∠XSR = ∠QSR ∼= ∠SQP = ∠XQP.
Moreover, Theorem 14 tells us that P Q = SR. Together with the underlined angle equalities and ASA, we conclude that ∆XSR ∼= ∆XQP (7)
from which it follows that P X = RX and QX = SX.
Theorem 16 (EG 28). The diagonals of parallelogram P QRS meet at a right angle if and only if the parallelogram is a rhombus.
Proof. Part I: Suppose that the diagonals P R, QS meet at a right angle. Then
P X = P X, QX = SX, and ∠P XS ∼= ∠P XQ,
the second equality by Theorem 15 and the third by Theorem 1. So ∆P XS ∼= ∆P XQ by SAS, and in particular P S = P Q. By the same argument, P Q = QR = RS = SP. That is, the parallelogram is a rhombus.
Part II: Suppose that the parallelogram is a rhombus. Then the triangles
∆P XQ, ∆RXQ, ∆RXS, ∆P XS (8)
are mutually congruent by SSS. In particular,
∠P XQ ∼= ∠RXQ ∼= ∠RXS ∼= ∠P XS.
But these four angles add up to 360◦^ by Axiom 7, so each of the four must equal 90◦.
Theorem 17 (EG 29). The diagonals of a parallelogram are congruent to each other if and only if the parallelogram is a rectangle.
Now we can start proving some theorems.
Theorem 18. Let ♦P QRS be a parallelogram^3. Then area(P QRS) = bh. where b = AB (the base) and h is the distance between P Q and RS; that is, the length of a line segment perpendicular to, and with endpoints on, P Q and RS.
Proof. First, if ♦P QRS happens to be a rectangle, then the theorem is immediate from Axiom 18. Otherwise, two opposite angles of the parallelogram are acute and two are obtuse. If necessary, swap the names P and Q, and R and S, so that without loss of generality ∠QRS is acute. Drop perpendiculars from P and Q to ←→ RS, meeting it at points T and U.
S U R
P Q
T
Then m∠P ST = m∠QRU (by Axiom 11) and m∠P T S = m∠QU R (Theorem 1) and therefore m∠SP T = m∠RQU (by Theorem 6). Moreover, P S = QR (because opposite sides of a parallelogram are equal, Theorem 14). By SAS it follows that ∆P T S ∼= ∆QU R. Now
area(♦P QRS) = area(∆QU R) + area(♦P SU Q) (by Axiom 17) = area(∆P T S) + area(♦P SU Q) (by Axiom 16) = area(♦P T U Q) (by Axiom 17) = P Q · P T = bh (by Axiom 18).
Theorem 19. A triangle with base b and height h has area bh/ 2.
Proof. Take a triangle ∆P QR. Construct a point S outside it such that ∆RSP ∼= ∆P QR, as shown. (You can do this by making a copy P ′Q′R′^ of the original triangle, and then placing P ′^ on Q and Q′^ on P .)
Q
S
P
R
By construction, m∠QP R = m∠SRP , so by Theorem 7, the lines
P Q and
RS are parallel. Simlarly, m∠QRP = m∠SP R, so by Theorem 7, the lines
QR and
P S are parallel. Therefore ♦P QRS is a parallel- ogram. Its base and height are that of the original triangle, so by Theorem 18 we have
area(♦P QRS) = bh = area(∆P QR) + area(∆RSP ).
On the other hand, area(∆P QR) = area(∆RSP ) by Axiom 16, so they both must equal bh/2.
(^3) Here and subsequently, the symbol ♦ means “quadrilateral”.
Then there’s the Pythagorean theorem (Theorem 12), which is proved earlier in these notes but does not make explicit reference to the axioms about area.
