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Postulates of Euclidean Geometry
Postulates 1–9 of Neutral Geometry.
Postulate 10E (The Euclidean Parallel Postulate). For each line and each point A that does not lie on , there is a unique line that contains A and is parallel to .
Postulate 11E (The Euclidean Area Postulate). For every polygonal region R, there is a positive real number S(R) called the area of R, which satisﬁes the following conditions:
(i ) (Area Congruence Property) If R1 and R2 are congruent simple regions, then S(R1) = S(R2). (ii ) (Area Addition Property) If R1, . . . , Rn are nonoverlapping simple regions, then S(R1 ∪ · · · ∪ Rn) =
S(R1) + · · · + S(Rn). (iii ) (Unit Area Property) If R is a square region with sides of length 1, then S(R) = 1.

Selected Theorems of Euclidean Geometry

All of the theorems of neutral geometry.

Theorem 10.1 (Converse to the Alternate Interior Angles Theorem). If two parallel lines are cut by a transversal, then both pairs of alternate interior angles are congruent.

Corollary 10.2 (Converse to the Corresponding Angles Theorem). If two parallel lines are cut by a transversal, then all four pairs of corresponding angles are congruent.

Corollary 10.3 (Converse to the Consecutive Interior Angles Theorem). If two parallel lines are cut by a transversal, then both pairs of consecutive interior angles are supplementary.

Lemma 10.4 (Proclus’s Lemma). Suppose and intersects , then t also intersects .

are parallel lines. If t is a line that is distinct from but

Theorem 10.5. Suppose and dicular to both.

are parallel lines. Then any line that is perpendicular to one of them is perpen-

Corollary 10.6. Suppose and are parallel lines, and m and m are distinct lines such that m ⊥ and m ⊥ . Then m m .

Corollary 10.7 (Converse to the Common Perpendiculars Theorem). If two lines are parallel, then they have a common perpendicular.

Theorem 10.8 (Converse to the Equidistance Theorem). If two lines are parallel, then each one is equidistant from the other.

Corollary 10.9 (Symmetry of Equidistant Lines). If and m are two distinct lines, then is equidistant from m if and only if m is equidistant from .

Theorem 10.10 (Transitivity of Parallelism). If , m, and n are distinct lines such that n.

m and m n, then

Theorem 10.11 (Angle-Sum Theorem for Triangles). Every triangle has angle sum equal to 180◦.

Corollary 10.12. In any triangle, the measure of each exterior angle is equal to the sum of the measures of the two remote interior angles.

Theorem 10.13 (60-60-60 Theorem). A triangle has all of its interior angle measures equal to 60◦ if and only if it is equilateral.

Theorem 10.14 (30-60-90 Theorem). A triangle has interior angle measures 30◦, 60◦, and 90◦ if and only if it is a right triangle in which the hypotenuse is twice as long as the shortest leg.

Theorem 10.15 (45-45-90 Theorem). A triangle has interior angle measures 45◦, 45◦, and 90◦ if and only if it is an isosceles right triangle.
Theorem 10.16 (Euclid’s Fifth Postulate). If and are two lines cut by a transversal t in such a way that the measures of two consecutive interior angles add up to less than 180◦, then and intersect on the same side of t as those two angles.
Theorem 10.17 (AAA Construction Theorem). Suppose AB is a segment, and α, β, and γ are three positive ←−−→
real numbers whose sum is 180. On each side of AB, there is a point C such that ABC has the following angle measures: m∠A = α◦, m∠B = β◦, and m∠C = γ◦.
Corollary 10.18 (Equilateral Triangle Construction Theorem). If AB is any segment, then on each side of
←−−→
AB there is a point C such that ABC is equilateral.
Theorem 10.19 (Angle-Sum Theorem for Convex Polygons). In a convex polygon with n sides, the angle sum is equal to (n − 2) × 180◦.
Corollary 10.20. In a regular n-gon, the measure of each angle is n−n 2 × 180◦. Corollary 10.21 (Exterior Angle Sum for a Convex Polygon). In any convex polygon, the sum of the measures of the exterior angles (one at each vertex ) is 360◦.
Theorem 10.22 (Angle-Sum Theorem for General Polygons). If P is any polygon with n sides, the sum of its interior angle measures is (n − 2) × 180◦.
Theorem 10.23 (Angle Sum Theorem for Quadrilaterals). Every convex quadrilateral has an angle sum of 360◦.
Corollary 10.24. A quadrilateral is equiangular if and only if it is a rectangle, and it is a regular quadrilateral if and only if it is a square.
Theorem 10.25. Every parallelogram has the following properties.
(a) Each diagonal cuts it into a pair of congruent triangles. (b) Both pairs of opposite sides are congruent. (c) Both pairs of opposite angles are congruent. (d ) Its diagonals bisect each other.
Theorem 10.26. If a quadrilateral has a pair of opposite sides that are both parallel and congruent, then it is a parallelogram.
Theorem 10.27. If a quadrilateral has a pair of opposite sides that are both perpendicular to a third side and congruent, then it is a rectangle.
Theorem 10.28 (Constructing a Rectangle). Suppose a and b are positive real numbers, and AB is a segment ←−−→
of length a. On either side of AB, there exist points C and D such that ABCD is a rectangle with AB = CD = a and AD = BC = b.
←−−→
Corollary 10.29 (Constructing a Square). If AB is any segment, then on each side of AB there are points C and D such that ABCD is a square.
Theorem 10.30 (Midsegment Theorem). Any midsegment of a triangle is parallel to the third side and half as long.
Chapter 11: Area
Lemma 11.1 (Convex Decomposition Lemma). Suppose P is a convex polygon, and BC is a chord of P. Then the two convex polygons P1 and P2 described in the polygon splitting theorem (Theorem 8.9 ) form an admissible decomposition of P, and therefore S(P) = S(P1) + S(P2).
Lemma 11.2. Suppose P is a convex polygon, O is a point in Int P, and {B1, . . . , Bm} are distinct points on P, ordered in such a way that for each i = 1, . . . , m, the angle ∠BiOBi+1 is proper and contains none of the Bj’s in its interior (where we interpret Bm+1 to mean B1). For each i = 1, . . . , m, let Pi denote the following set:
Pi = OBi ∪ OBi+1 ∪ P ∩ Int ∠BiOBi+1 .

Then each Pi is a convex polygon, and

S(P) = S(P1) + · · · + S(Pm).

(11.1)

Theorem 11.8 (Area of a Rectangle). The area of a rectangle is the product of the lengths of any two adjacent sides.

Lemma 11.9 (Area of a Right Triangle). The area of a right triangle is one-half of the product of the lengths of its legs.

Theorem 11.10 (Area of a Triangle). The area of a triangle is equal to one-half the length of any base multiplied by the corresponding height.

Corollary 11.11 (Triangle Sliding Theorem). Suppose ABC and A BC are triangles that have a common ←−−→
side BC, such that A and A both lie on a line parallel to BC. Then S ABC = S A BC .
Corollary 11.12 (Triangle Area Proportion Theorem). Suppose ABC and AB C are triangles with a common vertex A, such that the points B, C, B , C are collinear Then

S ABC = BC . S AB C B C

Theorem 11.13 (Area of a Trapezoid). The area of a trapezoid is the average of the lengths of the bases multiplied by the height.

Corollary 11.14 (Area of a Parallelogram). The area of a parallelogram is the length of any base multiplied by the corresponding height.

Chapter 12: Similarity

Theorem 12.1 (Transitivity of Similarity of Triangles). Two triangles that are both similar to a third triangle are similar to each other.

←−−→
Theorem 12.2 (The Side-Splitter Theorem). Suppose ABC is a triangle, and is a line parallel to BC that

intersects AB at an interior point D. Then also intersects AC at an interior point E, and the following proportions

hold:

=

and

=.

AB AC

DB EC

Theorem 12.3 (AA Similarity Theorem). If there is a correspondence between the vertices of two triangles such that two pairs of corresponding angles are congruent, then the triangles are similar under that correspondence.

Theorem 12.4 (Similar Triangle Construction Theorem). If ABC is a triangle and DE is any segment, ←−−→
then on each side of DE, there is a point F such that ABC ∼ DEF .

Theorem 12.5 (SSS Similarity Theorem). If ABC and DEF are triangles such that AB/DE = AC/DF = BC/EF , then ABC ∼ DEF .

Theorem 12.6 (SAS Similarity Theorem). If AB/DE = AC/DF , then ABC ∼ DEF .

ABC and

DEF are triangles such that ∠A ∼= ∠D and

Theorem 12.7 (Two Transversals Theorem). Suppose and are parallel lines, and m and n are two distinct transversals to and meeting at a point X that is not on either or . Let M and N be the points where m and n, respectively, meet ; and let M and N be the points where they meet . Then XM N ∼ XM N .

Theorem 12.8 (Converse to the Side-Splitter Theorem). Suppose ABC is a triangle, and D and E are interior points on AB and AC, respectively, such that

←−−→

←−−→

Then DE is parallel to BC.

AB AC

Theorem 12.9 (Angle Bisector Proportionality Theorem). Suppose ABC is a triangle and D is a point on BC that also lies on the bisector of ∠BAC. Then

BD AB =.
DC AC

Theorem 12.10 (Parallel Projection Theorem). Suppose , m, n, t, and t are distinct lines such that m n; t intersects , m, and n at A, B, and C, respectively; and t intersects the same three lines at A , B , and C , respectively. If B is between A and C, then B is between A and C , and

AB A B

=

.

BC B C

Theorem 12.11 (Menelaus’s Theorem). Let ABC be a triangle. Suppose D, E, F are points diﬀerent from

A, B, C and lying on such that either two of the points lie on ABC or none of them do. Then D, E, and F are

collinear if and only if

AD BE CF · · = 1.
DB EC F A

Theorem 12.12 (Ceva’s Theorem). Suppose ABC is a triangle, and D, E, F are points in the interiors of AB, BC, and CA, respectively Then the three cevians AE, BF , and CD are concurrent if and only if

AD BE CF · · = 1.
DB EC F A

Theorem 12.13 (Median Concurrence Theorem). The medians of a triangle are concurrent, and the distance from the point of intersection to each vertex is twice the distance to the midpoint of the opposite side.

Theorem 12.19 (Triangle Area Scaling Theorem). If two triangles are similar, then the ratio of their areas
is the square of the ratio of their corresponding side lengths; that is, if ABC ∼ DEF and AB = r · DE, then S ABC = r2 · S DEF .

Theorem 12.20 (Quadrilateral Area Scaling Theorem). If two convex quadrilaterals are similar, then the ratio of their areas is the square of the ratio of their corresponding side lengths.

Chapter 13: Right triangles
Theorem 13.1 (The Pythagorean Theorem). Suppose ABC is a right triangle with right angle at C, and let a, b, and c denote the lengths of the sides opposite A, B, and C, respectively. Then a2 + b2 = c2.
Theorem 13.2 (Converse to the Pythagorean Theorem). Suppose ABC is a triangle with side lengths a, b, and c. If a2 + b2 = c2, then ABC is a right triangle, and its hypotenuse is the side of length c. Theorem 13√.3 (Side Lengths of 30-60-90 Triangles). In a triangle with angle measures 30◦, 60◦, and 90◦, the longer leg is 3 times as long as the shorter leg, and the hypotenuse is twice as long as the shorter leg. Theorem 13.4 (Side Lengths of 45-45√-90 Triangles). In a triangle with angle measures 45◦, 45◦, and 90◦, the legs are congruent, and the hypotenuse is 2 times as long as either leg.
√ Theorem 13.5 (Diagonal of a Square). In a square, each diagonal is 2 times as long as each side.
Theorem 13.6 (SSS Existence Theorem). Suppose a, b, and c are positive real numbers such that each one is strictly less than the sum of the other two. Then there exists a triangle with side lengths a, b, and c.
Corollary 13.7 (SSS Construction Theorem). Suppose a, b, and c are positive real numbers such that each one ←−−→
is strictly less than the sum of the other two, and AB is a segment of length c. Then on either side of AB, there exists a point C such that ABC has side lengths a, b, and c opposite vertices A, B, and C, respectively.
Theorem 13.8 (Right Triangle Similarity Theorem). The altitude to the hypotenuse of a right triangle cuts it into two triangles that are similar to each other and to the original triangle.
Theorem 13.9 (Right Triangle Proportion Theorem). In every right triangle, the following proportions hold:
(a) The altitude to the hypotenuse is the geometric mean of the projections of the two legs onto the hypotenuse.

(b) Each leg is the geometric mean of its projection onto the hypotenuse and the whole hypotenuse.
Theorem 13.12 (The Pythagorean Identity). If θ is any real number in the interval [0, 180], then (sin θ)2 + (cos θ)2 = 1.
Theorem 13.13 (The Law of Cosines). Let ABC be any triangle, and let a, b, and c denote the lengths of the sides opposite A, B, and C, respectively. Then
a2 + b2 = c2 + 2ab cos ∠C.

Theorem 13.14 (Law of Sines). Let ABC be any triangle, and let a, b, and c denote the lengths of the sides opposite A, B, and C, respectively. Then

sin ∠A = sin ∠B = sin ∠C .

a

b

c

Theorem 13.15 (Heron’s Formula). Let ABC be a triangle, and let a, b, c denote the lengths of the sides opposite A, B, and C, respectively. Then

1
S ABC = (s(s − a)(s − b)(s − c)) 2

where s = (a + b + c)/2 (called the semiperimeter of ABC).
Theorem 13.16 (Sum Formulas). Suppose α and β are real numbers such that α, β, and α + β are all strictly between 0◦ and 90◦. Then

sin(α + β) = sin α cos β + cos α sin β, cos(α + β) = cos α cos β − sin α sin β.

Corollary 13.17 (Double Angle Formulas). Suppose α is a real number strictly between 0◦ and 45◦. Then

sin 2α = 2 sin α cos α, cos 2α = cos2 α − sin2 α.

Chapter: 14 Circles
Theorem 14.4 (Properties of Secant Lines). Suppose C is a circle and is a secant line that intersects C at A and B. Then every interior point of the chord AB is in the interior of C, and every point of that is not in AB is in the exterior of C.
Theorem 14.5 (Properties of Chords). Suppose C is a circle and AB is a chord of C.
(a) The perpendicular bisector of AB passes through the center of C.
(b) If AB is not a diameter, a radius of C is perpendicular to AB if and only if it bisects AB.
Theorem 14.6 (Line-Circle Theorem). Suppose C is a circle and is a line that contains a point in the interior of C. Then is a secant line for C, and thus there are exactly two points where intersects C.
Theorem 14.7 (Tangent Line Theorem). Suppose C is a circle, and is a line that intersects C at a point P . Then is tangent to C if and only if is perpendicular to the radius through P .
Corollary 14.8. If C is a circle and A ∈ C, there is a unique line tangent to C at A.
Theorem 14.9 (Properties of Tangent Lines). If C is a circle and is a line that is tangent to C at P , then every point of except P lies in the exterior of C, and every point of C except P lies on the same side of as the center of C. Lemma 14.15. Any two conjugate arcs have measures adding up to 360◦.
Theorem 14.16 (Another Thales’s Theorem). Any angle inscribed in a semicircle is a right angle.

Theorem 14.17 (Converse to Thales’s Theorem). The hypotenuse of a right triangle is a diameter of a circle that contains all three vertices.
Theorem 14.18 (Existence of Tangent Lines Through an Exterior Point). Let C be a circle, and let A be a point in the exterior of C. Then there are exactly two distinct tangent lines to C containing A. The two points of tangency X and Y are equidistant from A, and the center of C lies on the bisector of ∠XAY .
Theorem 14.19 (Inscribed Angle Theorem). The measure of a proper angle inscribed in a circle is one-half the measure of its intercepted arc.
Corollary 14.20 (Arc Addition Theorem). Suppose A, B, and C are three distinct points on a circle C, and AB and BC are arcs that intersect only at B. Then m ABC= m AB +m BC.
Corollary 14.21 (Intersecting Chords Theorem: Power of a Point). Suppose AB and CD are two distinct chords of a circle C that intersect at a point P ∈ Int C. Then

(P A)(P B) = (P C)(P D).

(14.2)

Corollary 14.22 (Intersecting Secants Theorem: Power of a Point). Suppose two distinct secant lines of a circle C intersect at a point P exterior to C. Let A, B be the points where one of the secants meets C, and C, D be the points where the other one does. Then

(P A)(P B) = (P C)(P D).

(14.3)

Theorem (Circumscribed circle theorem). For every triangle there exists a circumscribed cirlce: the circle that contans all three vertices of the triangle. The center of the circumscribed circle is the intersection point of the three perpendicular bisectors of the triangle.
Theorem (Inscribed circle theorem). For every triangle there exists an inscribed cirlce: the circle that is tangent to all three sides of the triangle. The center of the circumscribed circle is the intersection point of the three angle bisectors of the triangle.
Theorem 14.28 (Cyclic Quadrilateral Theorem). A quadrilateral ABCD is cyclic if and only if it is convex and both pairs of opposite angles are supplementary: m∠A + m∠C = 180◦ and m∠B + m∠D = 180◦.
Theorem (Concurrence theorems). Let ABC be a triangle.

1. The medians of ABC are concurrent. 2. The angle bisectors of ABC are concurrent. 3. The perpendicular bisectors of ABC are concurrent. 4. The lines containing the three altitudes of ABC are concurrent.

Chapter 16: Compass and Straightedge Constructions
Construction Problem 16.1 (Equilateral Triangle on a Given Segment). Given a segment AB and a side ←−−→
of AB, construct a point C on the chosen side such that ABC is equilateral.
Construction Problem 16.2 (Copying a Line Segment to a Given Endpoint). Given a line segment AB and a point C, construct a point X such that CX ∼= AB.
Construction Problem 16.3 (Cutting Oﬀ a Segment). Given two segments AB and CD such that CD > AB, construct a point E in the interior of CD such that CE ∼= AB.
Construction Problem 16.4 (Bisecting an Angle). Given a proper angle, construct its bisector.
Construction Problem 16.5 (Perpendicular Bisector). Given a segment, construct its perpendicular bisector.
Construction Problem 16.6 (Perpendicular Through a Point on a Line). Given a line and a point A ∈ , construct the line through A and perpendicular to .

Construction Problem 16.7 (Perpendicular Through a Point Not on a Line). Given a line and a point A ∈/ , construct the line through A and perpendicular to .

Construction Problem 16.8 (Triangle with Given Side Lengths). Given three segments such that the length of the longest is less than the sum of the lengths of the other two, construct a triangle whose sides are congruent to the three given segments.

Construction Problem 16.9 (Copying a Triangle to a Given Segment). Given a triangle ABC, a segment

←−−→
DE congruent to AB, and a side of DE, construct a point F on the given side such that

DEF ∼=

AB C .

Construction Problem 16.10 (Copying an Angle to a Given Ray). Given a proper angle ∠ab, a ray → −c , and a side of ← → c , construct the ray d→ − with the same endpoint as c→ − and lying on the given side of c← → such that ∠cd ∼= ∠ab.

Construction Problem 16.11 (Copying a Convex Quadrilateral to a Given Segment). Given a convex ←−−→
quadrilateral ABCD, a segment EF congruent to AB, and a side of EF , construct points G and H on the given side such that EF GH ∼= ABCD.

Construction Problem 16.12 (Rectangle with Given Side Lengths). Given any two segments AB and EF ,

and

a

side

of

←−−→
AB,

construct

points

C

and

D

on

the

given

side

such

that

AB C D

is

a

rectangle

with

BC

∼=

EF .

←−−→
Construction Problem 16.13 (Square on a Given Segment). Given a segment AB and a side of AB, construct points C and D on the chosen side such that ABCD is a square.

Construction Problem 16.14 (Parallel Through a Point Not on a Line). Given a line and a point A ∈/ , construct the line through A and parallel to .

Construction Problem 16.15 (Cutting a Segment into n Equal Parts). Given a segment AB and an integer n ≥ 2, construct points C1, . . . , Cn−1 ∈ Int AB such that A ∗ C1 ∗ · · · ∗ Cn−1 ∗ B and AC1 = C1C2 = · · · = Cn−1B.

Construction Problem 16.16 (Geometric Mean of Two Segments). Given two segments AB and CD, construct a third segment that is their geometric mean.

Construction Problem 16.17 (The Golden Ratio). Given a line segment AB, construct a point E ∈ Int AB such that AB/AE is equal to the golden ratio.

Construction Problem 16.18 (Parallelogram with the Same Area as a Triangle). Suppose ABC is a triangle and ∠rs is a proper angle. Construct a parallelogram with the same area as ABC, and with one of its angles congruent to ∠rs.

Construction Problem 16.19 (Rectangle with a Given Area and Edge). Given a rectangle ABCD, a ←−−→
segment EF , and a side of EF , construct a new rectangle with the same area as ABCD, with EF as one of its ←−−→
edges, and with its opposite edge on the given side of EF .

Construction Problem 16.20 (Squaring a Rectangle). Given a rectangle, construct a square with the same area as the rectangle.

Construction Problem 16.21 (Squaring a Convex Polygon). Given a convex polygon, construct a square with the same area as the polygon.

Construction Problem 16.22 (Doubling a Square). Given a square, construct a new square whose area is twice that of the original one.

Circle Constructions
Construction Problem 16.23 (Center of a Circle). Given a circle, construct its center. Construction Problem 16.24 (Inscribed Circle). Given a triangle, construct its inscribed circle. Construction Problem 16.25 (Circumscribed Circle). Given a triangle, construct its circumscribed circle.

Constructing Regular Polygons
Construction Problem 16.26 (Square Inscribed in a Circle). Given a circle and a point A on the circle, construct a square inscribed in the circle that has one vertex at A.
Construction Problem 16.27 (Regular Pentagon Inscribed in a Circle). Given a circle and a point A on the circle, construct a regular pentagon inscribed in the circle that has one vertex at A.
Construction Problem 16.28 (Regular Hexagon Inscribed in a Circle). Given a circle and a point A on the circle, construct a regular hexagon inscribed in the circle that has one vertex at A.
Construction Problem 16.29 (Equilateral Triangle Inscribed in a Circle). Given a circle and a point A on the circle, construct an equilateral triangle inscribed in the circle that has one vertex at A.
Construction Problem 16.30 (Regular Octagon Inscribed in a Circle). Given a circle and a point A on the circle, construct a regular octagon inscribed in the circle that has one vertex at A.
TriangleCircleAbcLinesSquare