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Theorems

Theorems

Terms

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Theorem 9
if two sides of a triangle are equal, the angles opposite them are equal
Theorem 13
if two sides of a triangle are unequal, the angles opposite them are unequal in the same order
Theorem 27
a quadrilateral is a parallelogram if its opposite sides are equal
Theorem 57
if a line through the center of a circle bisects a chord that is not a diameter, it is also perpendicular to the chord
Theorem 5 (linear pairs)
the angles in a linear pair are supplementary
Theorem 21
an exterior angle of a triangle is equal to the sum of the remote interior angles
Theorem 40
the area of a parallelogram is the product of any base and its corresponding altitude
Theorem 8 (linear pairs/perpendicular)
if the angles in a linear pair are equal, then then their sides are perpendicular
SSS Similarity Theorem
if the sides of one triangle are proportional to the sides of another triangle, then the triangles are similar
Theorem 15 (triangle inequality)
sum of any two sides of a triangle is greater than the third side
SAS Similarity Theorem
if an angle of one triangle is equal to an angle of another triangle and the sides including these angles are proportional, then the triangles are similar
Theorem 31
all rectangles are parallelograms
Theorem 41
the area of a trapezoid is half the product of its altitude and the sum of its bases
Theorem 25
the opposite sides and angles of a parallelogram are equal
Theorem 28
a quadrilateral is a parallelogram if its opposite angles are equal
Theorem 29
a quadrilateral is a parallelogram if two opposite sides are both parallel and equal
Theorem 3 (complements)
complements of the same angle are equal
Theorem 7 (perpendicular lines)
perpendicular lines form right angles
Theorem 37 (midsegment)
a midsegment of a triangle is parallel to the third side and half as long
Theorem 56
if a line through the center of a circle is perpendicular to a chord, it also bisects the chord
Theorem 60
if a line is perpendicular to a radius at its outer endpoint, it is tangent to the circle
Theorem 20 (triangle angle sum)
the sum of the angles of a triangle is 180 degrees
Theorem 2 (B.O.R.)
if OA-OB-OC, then
Theorem 51
In a 30-60-90 triangle, the hypotenuse is twice the shorter leg and the longer leg is the square root of 3 times the shorter leg
Theorem 6 (vertical angles)
vertical angles are equal
Theorem 24 (quadrilateral angle sum)
the sum of the angles of a quadrilateral is 360 degrees
Theorem 43 (converse of pythagorean)
if the square of one side of a triangle is equal to the sum of the squares of the other two sides, the triangle is a right triangle
Theorem 59
if a line is tangent to a circle, it its perpendicular to the radius drawn to the point of contact
Theorem 17
equal corresponding angles mean that lines are parallel
Theorem 16
in a plane, two points each equidistant from the endpoints of a line segment determine the perpendicular bisector of the line segment
Theorem 1 (B.O.P.)
if A-B-C, then AB+BC=AC
Theorem 14
if two angles of a triangle are unequal, the sides opposite them are unequal in the same order
Theorem 10
if two angles of a triangle are equal, the sides opposite them are equal
Theorem 26
the diagonals of a parallelogram bisect each other
Theorem 36
the diagonals of an isosceles trapezoid are equal
Theorem 53
two nonvertical lines are perpendicular iff the product of their slopes is -1
Theorem 12 (exterior angle)
an exterior angle of a triangle is greater than either remote interior angle
Theorem 42 (pythagorean)
the square of the hypotenuse of a right triangle is equal to the sum of the squares of its legs
Theorem 23 (hl)
if the hypotenuse and a leg of one right triangle are equal to the corresponding parts of another right triangle, the triangles are congruent
Theorem 32
all rhombuses are parallelograms
Theorem 46
corresponding altitudes of similar triangles have the same ratio as that of the corresponding sides
Theorem 19
parallel lines form equal corresponding angles
Theorem 49
the altitude to the hypotenuse of the a right triangle forms two triangles similar to it and to each other
Theorem 52
two nonvertical lines are parallel iff their slopes are equal
Theorem 18
in a plane, two lines parallel to a third line are parallel to each other
Theorem 11 (sss)
if the three side of one triangle are equal to three sides fo another triangle, the triangles are congruent
Theorem 55 (law of cosines)
if the sides opposite
Theorem 4 (supplements)
supplements of the same angle are equal
Theorem 54 (law of sines)
if the sides opposite
Theorem 33
the diagonals of a rectangle are equal
Theorem 45 (aa)
if two angles of one triangle are equal to two angles of another triangle, the triangles are similar
Theorem 35
the base angles of an isosceles trapezoid are equal
Theorem 30
a quadrilateral is a parallelogram if its diagonals bisect each other
Theorem 39
the area of a triangle is half the product of any base and its corresponding altitude
Theorem 34
the diagonals of a rhombus are perpendicular
Theorem 50 (Isosceles Right Triangle)
in an isosceles right triangle, the hypotenuse is the square root of two times the length of a leg
Theorem 48
the ratio of the areas of two similar polygons is equal to the square of the ratio of the corresponding sides
Theorem 22 (aas)
if two angles and the side opposite one of them in one triangle are equal to the corresponding parts of another right triangle, the triangles are congruent
Theorem 58
the perpendicular bisector of a chord of a circle contains the center of the circle
Theorem 44 (side splitter)
if a line parallel to one side of a triangle intersects the other two sides in different points, it divides the sides in the same ratio
Theorem 47
the ratio of the perimeters of two similar polygons is equal to the ratio of the corresponding sides
Theorem 38
the area of a right triangle is half the product of its legs

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