#2. Isosceles Triangle Theorem says that If a triangle is isosceles, then its BASE ANGLES are congruent.

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1 1 Geometry Proofs Reference Sheet Here are some of the properties that we might use in our proofs today: #1. Definition of Isosceles Triangle says that If a triangle is isosceles then TWO or more sides are #2. Isosceles Triangle Theorem says that If a triangle is isosceles, then its BASE ANGLES are #3. Definition of Acute Triangle/Definition of Obtuse Triangle says that If a triangle is an acute triangle, then all of its angles are less than 90 degrees. If a triangle is an obtuse triangle, then one of its angles is greater than 180 degrees. #4. Definition of Perpendicular says that If two lines, rays, segments or planes are perpendicular, then they form right angles (as many as four of them). #5. Definition of Right Angle/Definition of Acute Angle/Definition of Obtuse Angle says that: If an angle is a right angle, then the angle must EQUAL 90 degrees. If an angle is an acute angle, then the angle must be less than 90 degrees. If an angle is an obtuse angle, then the angle must be greater than 90 degrees. #6. ASA, SSS, SAS, AAS Proves that two triangles are congruent. ASA: says that If two angles and an included side of one triangle are congruent to two corresponding angles and an included side of another triangle, then the triangles are

2 2 SSS: says that If all three sides of one triangle are congruent to all three corresponding sides of a another triangle, then the triangles are SAS: says that If two sides and an included angle of one triangle are congruent to two corresponding sides and an included angle of another triangle, then the triangles are AAS: says that If two angles and a non-included sides of one triangle are congruent to two angles and a non-included side of another triangle, then the triangles are #7. CPCTC says that If you use ASA, SSS, SAS, or AAS to prove that two triangles are congruent, then all other corresponding parts (sides & angles) of the congruent triangles are going to be #8. Reflexive Property says that something is congruent to itself. #9. Segment Addition Postulate/ Angle Addition Postulate used when we do part + part = whole (for either sides or angles). #10. Definition of Vertical Angles says that If two non-adjacent angles are created by intersecting lines, then those angles are known as vertical angles. #11. Vertical Angle Theorem says that If two angles are vertical angles, then their measures are going to be congruent to one another.

3 3 #12. Definition of Linear Pair says that If two angles are adjacent and form a line, then they form what s known as a linear pair. #13. Linear Pair Postulate says that If two angles form a linear pair, then those angles are also going to be supplementary. #14. Definition of Supplementary/Definition of Complementary says that If two angles are supplementary, then their measures add up to 180 degrees. If two angles are complementary, then their measures add up to 90 degrees. #15. Definition of Parallel Lines says that If lines in the same plane do not intersect, then the lines are parallel. Angle relationships due to parallel lines #16. Definition of alternate interior angles says that If two angles are alternate interior, then they are on opposite sides of a transversal and are both on the interior to two lines (whether parallel or not). #17. Alternate interior angle theorem says that If two lines are parallel and alternate interior angles are formed, then the angles will be congruent to one another. #18. Converse of alternate interior angle theorem says that If alternate interior angles are congruent, then the lines that form them will be parallel to one another. #19. Definition of alternate exterior angles says that If two angles are alternate exterior, then they are on opposite sides of a transversal and are both on the exterior to two lines (whether parallel or not). #20. Alternate exterior angle theorem says that If two lines are parallel and alternate exterior angles are formed, then the angles will be congruent to one another. #21. Converse of alternate exterior angle theorem says that If alternate exterior angles are congruent, then the lines that form them will be parallel to one another.

4 4 #22. Definition of corresponding angles says that If two angles are corresponding, then they are on same side of a transversal and are both on corresponding sides (one interior/one exterior) to two lines (whether parallel or not). #23. Corresponding Angle Postulate says that If two lines are parallel and corresponding angles are formed, then the angles will be congruent to one another. #24. Converse of corresponding angle postulate says that If corresponding angles are congruent, then the lines that form them will be parallel to one another. #25. Definition of same side interior angles says that If two angles are same side interior, then they are on the same side of a transversal and are both on the interior to two lines (whether parallel or not). #26. Same side interior angle theorem says that If two lines are parallel and same side interior angles are formed, then the angles will be supplementary to one another. #27. Converse of same side interior angle theorem says that If same side interior angles are supplementary, then the lines that form them will be parallel to one another. #27. Definition of a midpoint says that If a point is a midpoint, then the point divides a segment into TWO equal parts. #28. Midpoint Theorem says that If a point is a midpoint, then the point divides a segment so that each part of the segment is equal to ONE HALF of the whole segment. #29. Definition of a Median If a segment is a median, then it is a segment whose endpoints are the vertex of a triangle and the midpoint of the opposite side of the triangle. #30. Definition of centroid says that If a point is a centroid, then it is a point of concurrency of the medians inside of a triangle. #31. Centroid Theorem says that If a point of concurrency of a triangle is a centroid, then the point that they are concurrent at is two thirds the distance from each vertex to the midpoint of the opposite side.

5 5 #32. Definition of Angle Bisector says that If a segment, ray, line or plane is an angle bisector, then it divides an angle into TWO equal parts. #33. Angle Bisector Theorem says that If a segment, ray, line or plane is an angle bisector, then it divides an angle so that each part of the angle is equal to ONE HALF of the whole angle. It also says: If a point is on the bisector of an angle, then the point is equidistance from the sides of the angle. #34. Hinge Theorem (SAS Inequality Theorem) #35. Converse of Hinge Theorem (SSS Inequality Theorem) #36. Isosceles Triangles Property Remember that the following things happen. The following will be the same segment: (Median; Altitude; Angle bisector) Vertex Angle Bisector Conjecture If a triangle is an isosceles triangle, then the median, angle bisector, and altitude will be the same segment. #37. Triangle Sum Theorem says that If a polygon is a triangle, then its interior angles will measure a sum of 180 degrees. #38. Definition of equidistant says that If a point is equidistant from two other points (or objects), then it is the same distance from the other two points (or objects). #39. Definition of Segment Bisector says that If a segment, ray, line or plane is a segment bisector, then it divides an segment into TWO equal parts.

6 6 #40. Transitive Property says that If one expression is equal/congruent to a second expression, and that second expression is equal/congruent to a third expression, then the first and third expressions are also equal/ #41. Substitution Property says that If we insert an expression into an equation in place of another expression, then we have used substitution. #42. Addition/Subtraction/Division/Multiplication says that If we add, subtract, multiply, or divide an number on BOTH sides of the equal sign in an equation, then we have carried out one of those basic arithmetic operations. #43. Distributive Property says that If we multiply items on the same side of the equal sign of an equation in which parentheses are involved, then we have used the distributive property. #44. Combining Like Terms says that If we add or subtract expressions on the same side of the equal sign, then we have combined like terms. #45. Exterior Angle Theorem says that If we have an exterior angle of a triangle, then its measure will equal the sum of its two remote interior angles. #46. Definition of Altitude says that If a segment is an altitude, then it is a segment originating from one of the vertices of a triangle and its perpendicular to an opposite side.