# Geometry Conjectures

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As we encounter conjectures in the text, we will be adding them to our set of terms to know.

### Linear Pair Conjecture C-1

If two angles form a linear pair, then the measures of the angles add up to 180 degrees.

### Vertical Angle Conjecture C-2

If two angles are vertical angles, then they have equal measures (are congruent).

### Parallel Lines Conjecture C-3

If two parallel lines are cut by a transversal, then corresponding angles are congruent, alternate interior angles are congruent and alternate exterior angles are congruent.

### Converse of the Parallel Lines Conjecture C-4

If two lines are cut by a transversal to form pairs of congruent corresponding angles congruent alternate interior angles or congruent alternate exterior angles then the lines are parallel.

### Perpendicular Bisector Conjecture C-5

If a point is on the perpendicular bisector of a segment, then it is equidistant from the endpoints.

### Converse of the Perpendicular Bisector Conjecture C-6

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

### Shortest Distance Conjecture C-7

the shortest distance from a point to a line is measured along the perpendicular segment from the point to the line.

### Angle Bisector Conjecture C-8

if a point is on the bisector of an angle, then it is equidistant from the sides of the angle.

### Parallel Slope Property

In a coordinate plane, two distinct lines are parallel if and only if their slopes are equal.

### Perpendicular Slope Property

In a coordinate plane, two nonvertical lines are perpendicular if and only if their slopes are negative reciprocals of each other.

### Angle Bisector Concurrency Conjecture C-9

The three angle bisectors of a triangle meet at a point (are concurrent).

### Perpendicular Bisector Concurrency Conjecture C-10

The three perpendicular bisectors of a triangle are concurrent.

### Altitude Concurrency Conjecture C-11

The three altitudes (or the lines containing the altitudes) of a triangle are concurrent.

### Circumcenter Conjecture C-12

The circumcenter of a triangle is equidistant from the vertices.

### Incenter Conjecture C-13

The incenter of a triangle is equidistant from the sides.

### Median Concurrency Conjecture C-14

the three medians of a triangle are concurrent.

### Centroid Conjecture C-15

The centroid of a triangle divides each median into two parts so that the distance from the centroid to the vertex is twice the distance from the centroid to the midpoint of the opposite side.

### Center of Gravity Conjecture C-16

The centroid of a triangle is the center of gravity of the triangular region.

### Euler Line Conjecture

The circumcenter, centroid, and orthocenter are the three points of concurrency that always lie on a line.

### Euler Segment Conjecture

The centroid divides the Euler Segment into two parts so that the smaller part is half the larger part.

### Triangle Sum Conjecture C-17

The sum of the measures of the angles in every triangle is 180 degrees

### Third Angle Conjecture C-18

If two angles of any one triangle are equal in measure to two angles of another triangle, then the third angle in each triangle is equal in measure to the third angle in the other triangle.

### Isosceles Triangle Conjecture C-19

If a triangle is isosceles then its base angles are congruent.

### Converse of the Isosceles Triangle Conjecture C-20

If a triangle has two congruent angles then it is an isosceles triangle.

### Triangle Inequality Conjecture C-21

The sum of the lengths of any two sides of a triangle is greater than the length of the third side.

### Side-Angle Inequality Conjecture C-22

In a triangle, if one side is longer than another side, then the angle opposite the longer side is larger than the angle opposite the shorter side.

### Triangle Exterior Angle Conjecture C-23

The measure of an exterior angle of a triangle is equal to the sum of the measures of the remote interior angles.
.

### SSS Congruence Conjecture C-24

If the three sides of one triangle are congruent to the three sides of another triangle, then the triangles are congruent.

### SAS Congruence Conjecture C-25

If two sides and the included angle of one triangle are congruent to two sides and the included angle of another triangle, then the triangles are congruent.
.

### ASA Congruence Conjecture C-26

If two angles and the included side of one triangle are congruent to two angles and the included side of another triangle, then the triangles are congruent.

### SAA Congruence Conjecture C-27

If two angles and a non-included side of one triangle are congruent to the corresponding angles and side of another triangle, then the triangles are congruent.

### Vertex Angle Bisector Conjecture C-28

In an isosceles triangle, the bisector of the vertex angle is also the altitude and the median to the base.

### Equilateral/Equiangular Triangle Conjecture C-29

Every equilateral triangle is equiangular, and, conversely, every equiangular triangle is
equilateral.

The sum of the measures of the four angles of any quadrilateral is 360 degrees

### Pentagon Sum Conjecture C-31

The sum of the measures of teh five angles of any pentagon is 540 degrees.

### Polygon Sum Conjecture C-32

The sum of the measures of the n interior angles of an n-gon is 180(n-2) degrees.

### Exterior Angle Sum Conjecture C-33

For any polygon, the sum of the measures of a set of exterior angles is 360 degrees.

### Equiangular Polygon Conjecture C-34

You can find the measure of each interior angle of an equiangular n-gon by using either of these formulas. 180-(360/n) or (180(n-2))/n

### Kite Angles Conjecture C-35

The nonvertex angles of a kite are congruent.

### Kite Diagonals Conjecture C-36

The diagonals of a kite are perpendicular.

### Kite Diagonal Bisector Conjecture C-37

The diagonal connecting the vertex angles of a kite is the perpendicular bisector of the other diagonal.

### Kite Angle Bisector Conjecture C-38

The vertex angles of a kite are bisected by a diagonal.

### Trapezoid Consecutive Angles Conjecture C-39

The consecutive angles between the bases of a trapezoid are supplementary.

### Isosceles Trapezoid Conjecture C-40

The base angles of an isosceles trapezoid are congruent.

### Isosceles Trapezoid Diagonals Conjecture C-41

The diagonals of an isosceles trapezoid are congruent.

### Three Midsegments Conjecture C-42

The three midsegments of a triangle divide it into four congruent triangles.

### Triangle Midsegment Conjecture C-43

A midsegment of a triangle is parallel to the third side and half the length of the third side.

### Trapezoid Midsegment Conjecture C-44

The midsegment of a trapezoid is parallel to the bases and is equal in length to the average of the lengths of the bases.

### Parallelogram Opposite Angles C-45

the opposite angles of a parallelogram are congruent.

### Parallelogram Consecutive Angles Conjecture C-46

the consecutive angles of a parallelogram are supplementary.

### Parallelogram Opposite Sides Conjecture C-47

The opposite sides of a parallelogram are congruent.

### Parallelogram Diagonals Conjecture C-48

the diagonals of a parallelogram bisect each other.

### Double-Edged Straightedge Conjecture C-49

If two parallel lines are intersected by a second pair of parallel lines that are the same distance apart as the first pair, then the parallelogram formed is a rhombus.

### Rhombus Diagonals Conjecture C-50

The diagonals of a rhombus are perpendicular, and they bisect each other.

### Rhombus Angles Conjecture C-51

The diagonals of a rhombus bisect the angles of the rhombus.

### Rectangle Diagonals Conjecture C-52

The diagonals of a rectangle are congruent and bisect each other.

### Square Diagonals Conjecture C-53

The diagonals of a square are congruent, perpendicular and bisect each other.

### Chord Central Angles Conjecture C-54

If two chords in a circle are congruent, then they determine two central angles that are congruent

### Chord Arcs Conjecture C-55

If two chords in a circle are congruent, then their intercepted arcs are congruent

### Perpendicular to a Chord Conjecture C-56

The perpendicular from the center of a circle to a chord is the bisector of the chord.

### Chord Distance to Center Conjecture C-57

Two congruent chords in a circle are equidistant from the center of the circle.

### Perpendicular Bisector of a Chord Conjecture

The perpendicular bisector of a chord passes through the center of the circle.

### Tangent Conjecture C-59

A tangent to a circle is perpendicular to the radius drawn to the point of tangency

### Tangent Segments Conjecture C-60

Tangent segments to a circle from a point outside the circle are congruent.

### Inscribed Angle Conjecture C-61

The measure of an angle inscribed in a circle is one-half the measure of the central angle.

### Inscribed Angles Intercepting Arcs Conjecture C-62

Inscribed angles that intercept the same arc are congruent.

### Angles Inscribed in a Semicircle Conjecture C-63

Angles inscribed in a semicircle are right angles.

The opposite angles of a cyclic quadrilateral are supplementary.

### Parallel Lines Intercepted Arcs Conjecture C-65

Parallel lines intercept congruent arcs on a circle.

### Circumference Conjecture C-66

If C is the circumference and d is the diameter of a circle, then there is a number such that C=πd. If d=2r where r is the radius, then C=2πr.

### Arc Length Conjecture C-67

the length of an arc equals the measure of the arc divided by 360 degrees times the circumference

### Reflection Line Conjecture C-68

the line of reflection is the perpendicular bisector of every segment joining a point in the original figure with its image

### Coordinate Transformations Conjecture C-69

The ordered pair rule (x, y) → (-x, y) is a reflection over the y-axis. The ordered pair rule (x, y) → (x, -y) is a reflection over the x-axis. The ordered pair rule (x, y) → (-x, -y) is a rotation about the origin. The ordered pair rule (x, y) → (y, x) is a reflection over y = x. (Lesson 7.2)

### Minimal Path Conjecture C-70

If points A and B are on one side of line l, then the minimal path from point A to line l to point B is found by reflecting B over line l and drawing segment AB'. The segment from A to the intersection of AB' and line l to B is the shortest path.

### Reflections over parallel lines Conjecture

A composition of two reflections over two parallel lines is equivalent to a single translation. in addition, the distance from any point to its second image under the two reflections is twice the distance between the parallel lines.

### Reflections over intersecting lines Conjecture

A composition of two reflections over a pair of intersecting lines is equivalent to a single rotation. The angle of rotation is twice the acute angle between the pair of intersecting reflection lines.

### Tessellating Triangles Conjecture C-73

Any triangle will create a monohedral tessellation. (Lesson 7.5)

Any quadrilateral will create a monohedral tessellation. (Lesson 7.5)

### Rectangle Area Conjecture C-75

The area of a rectangle is given by the formula A=bh, where A is the area, b is the lengh of the base, and h is the height of the rectangle

### Parallelogram Area Conjecture

The area of a parallelogram is given by the formula A=bh, where A is the area, b is the length of the base, and h is the length of the height of the parallelogram.

### Triangle Area Conjecture C-77

the area of a triangle is given by the formula A=1/2bh where A is the area, b is the length of the base, and h is the height of the triangle

### Trapezoid Area Conjecture

the area of a trapezoid is given by the formula A=h(b1+b2)/2 where A=area, b1+b2=length of bases, and h=height

### Kite Area Conjecture C-79

the area of a kite is given by the formula A=1/2d1d2 where d1 and d2 are the lengths of the diagonals

### Regular Polygon Area Conjecture

the area of a regular polygon is given by the formula A=1/2asn where A is the area, a is the apothem, s is the length of each side, and n is the number of sides. The length of each side times the number of sides is the perimeter P, so sn=P. Thus you can also write the formula for area as A=Pa/2.

### Pick's Formula

A=b/2+i-1
b=number of dots on the boundary of the polygon
i=number of dots in the interior of the polygon
A=area of the polygon

### Circle Area Conjecture C-81

the area of a circle is given by the formula A=∏r2 where A is the are and r is the radius of the circle

### Steps for finding Surface Area

1. Draw and label

a²+b²=c²

### Converse of the Pythagorean Theorem

If the lengths of the three sides of a triangle satisfy the Pythagorean equation, then the triangle is a right triangle.

### Isosceles Right Triangle Conjecture

In an isosceles right triangle, if the legs have length L, then the hypotenuse has length L√2

### 30-60-90 Degrees Triangle Conjecture

In a 30-60-90 degree triangle, if the shorter leg has length a, then the longer leg has length a √3 and the hypotenuse has length 2a.

### Distance Formula

d = √[( x₂ - x₁)² + (y₂ - y₁)²]. The distance between points A(x1,y1) and B(x2,y2)

### Equation of a circle

(x-h)²+(y-k)²=r². where r is radius and center is point (h,k).

### Volume of prism or cylinder C-88

V=Bh. where B is the area of the base of a right rectangular prism and h is the height of the solid.

### Pyramid-Cone Volume Conjecture C-89

if B is the area of the base of a pyramid (or cone) and H is the height of the solid, then the formula for the volume is V=⅓BH

### Sphere Volume Conjecture C-90

the volume of a sphere with radius r is given by the formula V(sphere)=4/3πr³

### Sphere Surface Area Conjecture C-91

the surface area, SA, of a sphere with radius r is given by the formula SA(sphere)=4πr²

### Dilation Similarity Conjecture C-92

if one polygon is the image of another polygon under a dilation, then the polygons are similar

### AA Similarity Conjecture

If two angles in a triangle are congruent to two angles in another triangle, then the triangles are similar.

### SSS Similarity Conjecture C-94

if the 3 sides of one triangle are proportional to the 3 sides of another triangle, then the triangles are similar

### SAS Similarity Conjecture C-95

if 2 sides of one triangle are proportional to 2 sides of another triangle and the included angles are congruent, then the triangles are similar

### Proportional Parts Conjecture C-96

if 2 triangles are similar, then the lengths of the corresponding altitudes, medians, and angle bisectors are proportional to the lengths of the corresponding sides

### Angle Bisector/Opposite Side Conjecture

A bisector of an angle in a triangle divides the opposite side into two segments whose lengths are in the same ratio as the lengths of the two sides forming the angle.

### Proportional Areas Conjecture

if corresponding sides of two similar polygons or the radii of two circles compare in the ratio m/n, then their areas compare in the ratio m²/n²

### Proportional Volumes Conjecture C-99

if corresponding edges or radii or heights of two similar solids compare in the ratio m/n, then their volumes compare in the ratio m³/n³

### Parallel/Proportionality Conjecture

if a line parallel to one side of a triangle passes through the other 2 sides then it divides the other 2 sides proportionally. Conversely, if a line cuts 2 sides of the triangle proportionally, then it is parallel to the 3rd side

### Extended Parallel/Proportionality Conjecture. C-101

If two or more lines pass through two sides of a triangle parallel to the third side, then they divide the two sides proportionally.

Example: