Goal • Use theorems about isosceles and equilateral triangles.
Vocabulary
When an isosceles triangle has exactly two congruent sides, these two sides are the legs. The angle formed by the legs is the vertex angle. The third side is the base of the isosceles triangles. The two angles adjacent to the base are called base angles.
Legs The legs of an isosceles triangle are the two congruent sides.
Vertex angle The vertex angle of an isosceles triangle is the angle formed by the legs.
Base The base of an isosceles triangle is the side that is not a leg.
Base angles The base angles of an isosceles triangle are the two angles adjacent to the base.
Once you have proved that two triangles are congruent, you know that their corresponding parts must be congruent as well. This will allow you to find the unknown measures of angles , objects and distances, as well as prove that constructions of copying angles are valid.
Goal: Use congruent triangles to prove corresponding parts congruent.
Directions: When attempting to prove triangles congruent, it is important to satisfy all of the conditions of the congruent triangle method you are using. This activity is designed to help you organize your thinking about how the parts of a congruent triangle proof will come together. In each problem below, examine the diagram and the GIVEN information. You may wish to draw the diagrams on paper so that you can mark off the information.
Determine the method needed to prove the triangles congruent. (ASA, SAS, AAS, SSS, or HL for right triangles only)
Check to see if you have the correct method by looking at the Method for Congruent Triangles box at the bottom of the chart.
Each of the three components needed to support the chosen method appear to the left of their corresponding Statement.
Decide what Reasons can be used to support your decisions.
In summary, when working with congruent triangles, remember to:
Mark any given information on your diagram.
Look to see if the pieces you need are "parts" of the triangles that can be proven congruent.
If not given all needed pieces to prove the triangles congruent, look to see what else you might know about the diagram.
Know your definitions! If the given information contains definitions, consider these as "hints" to the solution and be sure to use them.
Stay open-minded. There may be more than one way to solve a problem.
Look to see if your triangles "share" parts. These common parts are automatically one set of congruent parts.
Remember that proving triangles congruent is like solving a puzzle. Look carefully at the "puzzle" and use all of your geometrical strategies to arrive at an answer.
Goal • Identify angle pairs formed by three intersecting lines.
VOCABULARY 1.)Parallel lines Two lines are parallel lines if they do not intersect and are coplanar.
2.)Skew lines Two lines are skew lines if they do not intersect and are not coplanar.
3.)Parallel planes Two planes that do not intersect are parallel planes.
4.)Transversal A transversal is a line that intersects two or more coplanar lines at different points. 5.)Corresponding angles Two angles are corresponding angles if they have corresponding positions.
6.)Alternate interior angles Two angles are alternate interior angles if they lie between the two lines and on opposite sides of the transversal.
7.)Alternate exterior angles Two angles are alternate exterior angles if they lie outside the two lines and on opposite sides of the transversal.
8.)Consecutive interior angles Two angles are consecutive interior angles if they lie between the two lines and on the same side of the transversal.
Goal: Use algebraic properties in logical arguments.
Key Concept
You can use properties of real numbers to write logical arguments about geometric figures like angle measures and segment lengths.
Those that can be applied include the algebraic properties of equality (addition, subtraction, multiplication, division, and substitution), the distributive property and the reflexive, symmetric, and transitive properties of equality (for real numbers, segment length, and angle measure).
Example1: Write reasons for each step.
Example2: Use properties of equality for angles Example3: Use properties of equality for segments