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1 Lesson Plan #013 Class: Geometry Date: Friday October 7th, 2016 Topic: Deductive Reasoning in Geometry Aim: How do we use deductive reasoning to prove statements in geometry? HW # 013: Page 35 #’s 11, 15, 20, 21 Objectives: 1) Students will be able to prove statements using the reflexive, symmetric and transitive properties of equality. Do Now: We stated that a triangle that has two congruent sides is an isosceles triangle. construct two different isosceles triangles First Isosceles Triangle Using your compass and straight-edge, Second Isosceles Triangle In each isosceles triangle, use a protractor to measure the base angles. Based on this exercise, what can you state about the base angles of an isosceles triangle? PROCEDURE: Write the Aim and Do Now Get students working! Take attendance Give Back HW Collect HW Go over the Do Now Assignment #1: Let’s go to http://www.mathopenref.com/isosceles.html and take a look at isosceles triangles and their base angle measures. In the Do Now, we looked at some examples and then made a general truth based on those examples. That type of reasoning is called inductive reasoning. Use inductive reasoning to find the next term in the pattern Inductive reasoning is good, but it has its drawbacks. With inductive reasoning, you are making conclusions without examining every possible example. Any single counterexample is sufficient to show that a general conclusion reached is false. For example, if I measured all the angles of many isosceles triangles and found that all the angles I measured are acute angles, I would conjecture that all angles of isosceles triangles are acute. Can you show a counter example to show my conclusion is false? 2 Sample Test Questions: 1) 2) 3) Instead of using inductive reasoning, if we instead use definitions, laws, rules, formulas, theorems and other statements assumed to be true (postulates) to arrive at a true conclusion, then we are using deductive reasoning. For example, is an example of deductive reasoning! Which Law was used to arrive at the above true conclusion? Assignment #2: Determine if the following examples of reasoning are inductive or deductive. Sample Test Question: 3) 3 Sample Test Question: 4) Example #1:A demonstration of proving a statement using deductive reasoning. For example, Given: M is the midpoint of AB Prove: AM=BM Statements 1) M is the midpoint of AB Reasons 1. Given A midpoint divides a line segment into 2 congruent segments. (1) Congruent segments are equal in length. (2) 2) AM BM 2. 3) AM BM 3. In the proof above, the premises of the argument must be taken as true statements. In geometry, there are statements that are made that are neither undefined terms (such as point, line) nor definitions (a triangle is a polygon that has exactly 3 sides). Definition: A postulate is a statement whose truth is accepted without proof. Definition: A theorem is a statement that is proved by deductive reasoning. Let’s examine some postulates and see how they are used in proofs. The following 3 equality postulates are also referred to as the properties of equality. The Reflexive Property of Equality a a A quantity is equal to itself The Symmetric Property of Equality If a b , then b a The Transitive Property of Equality If a b and b c , then a c 4 Write proofs indicated below: 1. Statements 1.CD = 2 inches 2. XY= 2 inches 3.CD =XY Do proofs 2 and 3 on your own. Reasons 1. Given 2. Given 3.Transitive Property of Equality (1,2)