Betweeness Theorem: If B is between A and C, then AB+BC=AC 

Linear Pair Theorem: If two angles form a linear
pair, then they are supplementary 

Vertical Angle Theorem: If two angles are vertical angles, they have 
equal measures 

Parallel Lines and Slopes Theorem: Two nonverical lines are parallel 
if and only if they have the same slope 

Trasitivity of Parallelism Theorem: In a plane, if L is parallel to M,
and M is parallel to N, then L is parallel to N 

Two Perpendiculars Theorem: If two coplanar lines L and M are each 
perpendicular to the same line, then they are parallel to each other 

Perpendicular to Parallels Theorem: In a plane, if a line is 
perpendicular to one of two parallel lines, then it is perpendicular 
to the other 

Perpendicular Lines and Slopes Theorem: Two nonvertical lines are 
perpendicular if and only if the product of their slopes is -1 

Figure Reflection Theorem: If a figure is determined by certain points,
then its reflection image is the corresponding figure determined by 
the reflection images of those points 

Perpendicular Bisector Theorem: If a point is on the perpendicular 
bisector of a segment, then it is equidistant from the endpoints of 
the segment 

Flip-Flop Theorem: If F and F' are points or figures and r(F)=F', then
r(F')=F 

Segment Symmetry Theorem: A segment has exactly two symmetry lines: 
its perpendicular bisector and its line containing the segment 

Side Switching Theorem: If one side of an angle is reflected the line
containing the angle bisector, its image is the other side of the 
angle 

Angle Symmetry Theorem: The line containing the bisector of the angle
is a symmetry line of the angle 

Isosceles Triangle Symmetry Theorem: The line containing the bisectors 
of the vertex angle of an isosceles triangle is a symmetry line for 
the triangle 

Isosceles Triangle Theorem: If a triangle has two equal sides, then 
the angles opposite them are equal 

Kite Symmetry Theorem: The line containing the ends of a kite is a 
symmetry line for the kite 

Kite Diagonal Theorem: The symmetry diagonal of a kite is the 
perpendicular bisector of the other diagonal and bisects the two 
angles at the ends of the kite 

Rhombus Symmetry Theorem: Every rhombus has two symmetry lines: the 
bisectors of its diagonals 

Trapezoid Angle Theorem: In a trapezoid consecutive angles between a 
pair of parallel sides are supplementary 

Isosceles Trapezoid Symmetry Theorem: The perpendicular bisector of 
one base of an isosceles trapezoid is the perpendicular bisector of 
the other base and a symmetry line for the trapezoid 

Isosceles Trapezoid Theorem: In an isosceles trapezoid, the non-base 
sides are equal in measure

Rectangle Symmetry Theorem: Every rectangle has two symmetry lines:
the perpendicular bisectors of its bases

Parallel Lines/Alternate Interior Angles Theorem: If two parallel 
lines are cut by a transversal, then alternate interior angles are 
equal in measure

Alternate Interior Angles/Parallel Lines Theorem: If two lines are cut 
by a transversal and form equal alternate interior angles, then the
lines are parallel

Quadrilateral Hierarchy Theorem: If a figure is of any type on the 
hierarchy, it is also of all types connected above it

Triangle-Sum Theorem: The sum of the measures of the angles of a 
triangle is 180 degrees

Quadrilateral-Sum Theorem: The sum of the measures of the angles of a 
convex quadrilateral is 360 degrees

Polygon-Sum Theorem: The sum of the measures of the angles of a convex
polygon of n sides is (n-2)180

ABCD Theorem: Every isometry preserves angle measure, betweenness, 
collinearity(lines), and distance(length of segments)

Segment Congruence Theorem: Two segments are congruent if and only if
they have the same length

Angle Congruence Theorem: Two angles are congruent if and only if they
have equal measures

CPCF Theorem: If two figures are congruent, then any pair of 
corresponding parts is congruent

SSS Congruence Theorem: If, in two triangles, three sides of one are 
congruent to three sides of the other, then the triangles are 
congruent

SAS Congruence Theorem: If, in two triangles, two sides and the 
included angle of one, are congruent to two sides and the 
included anle of the other, then the triangles are congruent

ASA Congruence Theorem: If, in two triangles, two angles and the 
included side of one, are congruent to two sides and the included 
angle of the other, then the triangles are congruent

AAS Congruence Theorem: If, in two triangle, two angles and a non-
included side of one, are congruent respectively to two angles and a 
non-included side of the other then the two triangles are congruent 

HL Congruence Theorem: If, in two right triangles, the hypotenuse and leg 
of one are congruent to the hypotenuse and leg of the other, then the two 
triangles are congruent 

SsA Congruence Theorem: If, in two triangles, two sides and the angle
opposite the longer of the two sides in one are congruent respectively
to two sides and the angle opposite the longer of the two sides in the
other, then the two triangles are congruent.

Properties of a Parallelogram Theorem: In any parallelogram each 
diagonal forms two congruent triangles, opposite sides are congruent, 
and the diagonals intersect at their midpoints

Center of a Regular Polygon Theorem: In any regular polygon there is a point (its center) that is equidistant from all its verticies

Sufficient Conditions for a Parallelogram Theorem: If, in a quadrilateral: (a) both pairs of opposite sides are congruent, or (b) both pairs of opposite angles are congruent, or (c) the diagonals bisect each other, or (d) one pair of sides is parallel and congruent, then the quadrilateral is a parallelogram

SAS Inequality Theorem: If two sides of a triangle are congruent to two sides of a second triangle, and the measure of the included angle of the first triangle is less than the measure of the included angle of the second, then the third side of the first triangle is shorter than the third side of the second

Pythagorean Theorem: In any right triangle with legs a and b and hypotenuse c, a(sq) + b(sq) = c(sq)

Pythagorean Converse Theorem: If a triangle has sides of lengths a, b, and c, and a(sq) + b(sq) = c(sq), then the triangle is a right triangle

Midpoint Connector theorem: The segment connecting the midpoints of two sides of a triangle is parallel to and half the length of the third side.

Size Change Distance Theorem: Under a size change with magnitude k > 0, the distance between any two image points is k times the distance between their preimages

Size Change Theorem: Under a size transformation: angles and their measures are preserved: betweenness is preserved; collinearity is preserved: and lines and their images are parallel

Figure Size Change Theorem: If a figure is determined by certain points, then its size change image is the corresponding figure determined by the size change images of those points

Means-Extremes Property: If a/b = c/d, then ad = bc

Means Exchange Property: If a/b = c/d, then a/c = b/d

Reciprocals Property: If a/b = c/d, then b/a = d/c

Similar Figures Theorem: If two figures are similar, then:
Corresponding angles are congruent
Corresponding lengths are proportional

Fundamental Theorem of Similarity: If G ~ G’ and k is the ratio of similtude, then:
Perimeter (G’) = k times Perimeter (G)
Area (G’) = k(sq) times Area(G)
Volume (G’) = k(cu) times Volume (G)

SSS Similarity Theorem: If the three sides of one triangle are proportional to the three sides of a second triangle, then the triangles are similar

AA Similarity Theorem: If two triangles have two angles of one congruent to two angles of the other, then the triangles are similar

SAS Similarity Theorem: If, in two triangles, the ratios of two pairs of corresponding sides are equal and ith included angles are congruent, then the triangles are similar.

Side-Splitting Theorem: If a line intersects ray OP and ray OQ in distince points X and Y so that OX/XP = OY/YQ, then line XY is parallel to line PQ

Radius-Tangent Theorem: A line is tangent to a circle if and only if it is perpendicular to a radius at the radius’s endpoint on the circle

Uniqueness of Parallels Theorem: Through a point not on a line, there is exactly one parallel o the given line

Exterior Angle Theorem In a triangle, the measure of an exterior angle is equal to the sum of the measures of the two nonadjacent interior angles

Exterior Angle Inequality: In a triangle, the measure of an exterior angle is equal to the sum of the measures of the two nonadjacent interior angles

Unequal Sides Theorem: If two sides of a triangle are not congruent, then the angles opposite them are not congruent, and the larget angle is opposite the longer side

Unequal Angles Theorem: If two anles of a triangle are not congruent, then the sides opposite them are not congruent, and the longer side is opposite the larger angle

Exterior Angles of a Polygon Sum Theorem: In any convex polygon, the sum of the measures of the exterior angles, one at each vertex, is 360

Isosceles Right Triangle Theorem: In an isosceles right triangle, if a leg is x then the hypotenuse is x times the square root of 2

30-60-90 Triangle Theorem: In a 30-60-90 right triangle, if the short leg is x then the longer leg is x times the square root of 3 and the hypotenuse is 2x

Isoperimetric Theorem: Of all solids with the same volume, the sphere has the least surface area 

Isoperimetric Theorem: Of all solids with the same surface area, the sphere has the most volume

Isoperimetric Theorem: Of all plane figures withthe same area, the circle has the least perimeter

Isoperimetric Inequality: If a plane figure has area A and perimeter p, then A is greater then or equal to p(sq)/4(Pi)

Isoperimetric Theorem: Of all plane figures with the same perimeter, the circle has the most area

Tangent Square Theoerm: The power of point P for circle O is square of the length of a segment tangent to circle O

Secant Length Theorem: Suppose one secant intersects a circle at A and B, and a second secant intersects the circle at C and D. If the secants intersect at P, then AP times BP equals CP times DP

Tangent-Segment Theorem: The measure of an angle between two tangents, or between a tangent and a secant, is half the distance of the intercepted arcs

Tangent-Chord Theorem: The measure of an angle formed by a tangent and a chord is half  the measures of the  intercepted arc 

Angle-Secant Theorem: The measure of an angle formed by two secants intersecting outside a circle is half the difference of the arcs intercepted by it

Angle-Chord Theorem: The measure of an angle formed by two intersecting chords is one-half the sum of the measures of the arcs intercepted by it and its vertical angle

Inscribed Angle Theorem: In a circle the measure of an inscribed angle os one-half the measure of its intercepted arc

Arc-Chord Congruence Theorem: In a circle or in congruent circles: (a.) If two arcs have the same measure, they are congruent and their chords are congruent (b.) If two chords  have the same length, their minor arcs have the same measure

Chord-Center Theorem: (a.) The line containing the center of a circle perpendicular to a chord bisects the chord (b.) The line containing the center of a circle and the midpoint of a chord bisects the central angle determined by the chord (c.) The bisector of the central angle of a chord is perpendicular to the chord and bisects the chord (d.) The perpendicular bisector of a chord of a circle contains the center of the circle

Vector Addition Theorem: The sum of the vectors (a,b) and (c,d) is the vector (a +c, b +d)

Properties of Vector Addition Theorem: 
1. Vector addition is communative
2. Vector addition is associative  
3. (0,0) is an identity for vector addition
4. Every vector (a,b) has an additive inverse (-a,-b)

Right Triangle Altitude Theorem: In a right triangle:
The altitude to the hypotenuse is the geometric mean of the segments into which it divides the hypotenuse:
And each leg is the geometric mean of the hypotenuse and the segment of the hypotenuse adjacent to the leg