EXAMPLE 1 Use the Law of Detachment

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EXAMPLE 1 Use the Law of Detachment Use the Law of Detachment to make a valid conclusion in the true situation. If two segments have the same length, then they are congruent. You know that BC = XY. a. b. Mary goes to the movies every Friday and Saturday night. Today is Friday. SOLUTION Because BC = XY satisfies the hypothesis of a true conditional statement, the conclusion is also true. So, BC = XY. a.

EXAMPLE 1 Use the Law of Detachment b. First, identify the hypothesis and the conclusion of the first statement.The hypothesis is “If it is Friday or Saturday night,” and the conclusion is “then Mary goes to the movies.” “Today is Friday” satisfies the hypothesis of the conditional statement, so you can conclude that Mary will go to the movies tonight.

EXAMPLE 2 Use the Law of Syllogism If possible, use the Law of Syllogism to write a new conditional statement that follows from the pair of true statements. a. If Rick takes chemistry this year, then Jesse will be Rick’s lab partner.If Jesse is Rick’s lab partner, then Rick will get an A in chemistry. b. If x2 > 25, then x2 > 20. If x > 5, then x2 > 25. c. If a polygon is regular, then all angles in the interior of the polygon are congruent. If a polygon is regular, then all of its sides are congruent.

EXAMPLE 2 Use the Law of Syllogism SOLUTION The conclusion of the first statement is the hypothesis of the second statement, so you can write the following new statement. a. If Rick takes chemistry this year, then Rick will get an A in chemistry. b. Notice that the conclusion of the second statement is the hypothesis of the first statement, so you can write the following new statement. If x > 5, then x2 > 20.

EXAMPLE 2 Use the Law of Syllogism c. Neither statement’s conclusion is the same as the other statement’s hypothesis. You cannot use the Law of Syllogism to write a new conditional statement.

GUIDED PRACTICE for Examples 1 and 2 1. If 90° < m R < 180°, then R is obtuse. The measure of R is 155°. Using the Law of Detachment, what statement can you make? ANSWER R is obtuse

GUIDED PRACTICE for Examples 1 and 2 2. If Jenelle gets a job, then she can afford a car. If Jenelle can afford a car, then she will drive to school. Using the Law of Syllogism, what statement can you make ? ANSWER If Jenelle gets a job, then she will drive to school.

GUIDED PRACTICE for Examples 1 and 2 State the law of logic that is illustrated. 3. If you get an A or better on your math test, then you can go to the movies. If you go to the movies, then you can watch your favorite actor. If you get an A or better on your math test, then you can watch your favorite actor. ANSWER Law of Syllogism.

GUIDED PRACTICE for Examples 1 and 2 4. If x > 12, then x + 9 > 20. The value of x is 14 Therefore, x + 9 > 20 ANSWER Law of Detachment