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Mathematician: _________________________ Date: _______________ Core-Geometry: 4.3 Prove Triangles Congruent by SSS Warm-up: 1. Provide two linear equations that are parallel. 2. Provide an equation of a line that has an undefined slope. Review 1. Determine the distance between (10, -3) and (15, 5). 2. Determine the midpoint of AB where A is (5, 1) and B is (-1, -5). 1 Core Geometry: Investigation of Triangle Congruence Shortcuts If you prove that two triangles are congruent using the definition of congruence, then you need to show that all six parts (three angles and three sides) of both triangles are congruent. Fortunately there are “shortcuts”, and we can use less than six corresponding parts to prove two triangles are congruent. We will start by comparing just three parts of each triangle. 1. List the six possible combinations of three parts (angles and/or sides) in the first column of the table on the results sheet. Flipping the order creates identical possible combinations. For example, Angle – Angle – Side is the same as Side – Angle – Angle because they are the same three parts in reverse order. 2. Investigate each possible congruence shortcut by using the activity found on the following website: http://illuminations.nctm.org/Activity.aspx?id=3504 Instructions: Select three triangle parts from the top, right menu to start. If you choose side AB, angle A, and angle B, you will be working on Angle – Side – Angle. If instead you choose side AB, angle A, and angle C, you will be working on Angle – Angle – Side. This creates those parts in the work area. (Note: The tool does not allow you to select more than three parts. If you select the wrong part, simply unselect it before choosing another part.) Click and drag a dot to move the part to a new location. Click and drag a side's endpoint or angle's arrow to rotate the part. The center of rotation is the side's midpoint or the angle's vertex, respectively. Move the parts of the triangle so that points labeled with the same letter touch. To help place parts, points marked with the same letter snap together. When angles snap, the rays are extended to the edge of the work area. When you create a closed triangle, the points merge and center is filled in. Once a triangle is formed with the original three parts, the triangle moves to the bottom, right corner of the work area, and congruent parts appear. Form a second triangle with these congruent parts. After a second triangle is formed, you will be asked if they are congruent. You can test congruence by manipulating either triangle. Click and drag within the triangle to move it to a new location. Click and drag a vertex to rotate the triangle. Use the Flip button to reflect the triangle horizontally. First click on the triangle you would like to reflect, and then click the Flip button. i. If you can create two different triangles with the same parts, then those parts do not prove congruence. Careful - two triangles might be mirror images but still congruent, therefore you may have to flip your triangles to see how they are congruent. For example, all the triangles to the right are congruent. ii. If, however, the second triangle can only be formed congruent to the first, then that arrangement of three parts is a congruence shortcut. If the two triangles are congruent, you will be asked if it's possible to make a triangle that is not congruent to the original. If you create a third congruent triangle, you will be given the option to try again. The Reset button clears the work area and creates new sides and angles for the selected parts. The New button clears your selection and work area 2 Three Parts Name Does it prove congruence? 3 Sketch 4.3 Proving Triangles Congruent by SSS In the investigation you saw that there is only _______ way to form a triangle given _______________ side lengths. Any _________triangles with the same three _______________ lengths must be _______________________________. Example 1 Using the SSS Congruence Shortcut Decide whether the congruence statement is true. Explain your reasoning. a) b) Example 2 Triangles and Coordinate Geometry Using the coordinates listed, decide if the triangles below are congruent. If they are, write a congruence statement for the congruent triangles. Hmwk 3.2 p.236 Ex 4.3 # 1-9 odd, 17, 19 Quiz for lessons 4.1-4.3 # 1-5 4