2.2 Definitions and Biconditional Statements

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Presentation transcript:

2.2 Definitions and Biconditional Statements

Definitions Perpendicular Lines are those which intersect and form a right angle A line perpendicular to a plane is a line that intersects the plane in a point and is perpendicular to every line in the plane that intersects it. “is perpendicular to”

Goal 1:Recognizing and Using Definitions All definitions can be interpreted “forward” and “backward.” The definition of perpendicular lines means If two lines are perpendicular, then they intersect to form a right angle, and If two lines intersect to form a right angle, then they are perpendicular.

Example 1 Decide whether each statement about the diagram is true. Explain your answers. Points D, X, and B are collinear. TRUE Two points are collinear if they lie on the same line. AC is perpendicular to DB. TRUE The right angle symbol in the diagram indicates that lines AC and DB intersect to form a right angle; so the lines are perpendicular. AXB is adjacent to CXD. FALSE By definition, adjacent angles must share a common side. They do not, so are not adjacent. A D X B C

Goal 2:Using Biconditional Statements Conditional statements are not always written in the if-then form. Another type of a conditional is a biconditional statement in the only-if form. Here is an example. It is Saturday (hypothesis), only if I am working at the restaurant (conclusion). You can rewrite this biconditional statement in if-then form as follows: If it is Saturday, then I am working at the restaurant. A biconditional statement is one that contains the phrase “if and only if.” Writing a biconditional statement is equivalent to writing a conditional statement and its converse.

Example 2: Rewriting a Biconditional Statement The biconditional statement below can be rewritten as a conditional statement and its converse. Three lines are coplanar if and only if they lie in the same plane. Conditional statement: If three lines are coplanar, then they lie in the same plane. Converse: If three lines lie in the same plane, then they are coplanar.

Biconditional Statements A biconditional statement can either be true or false. To be true, BOTH the conditional statement and its converse must be true. This means that a true biconditional statement is true both “forward” and “backward.” All definitions can be written as true biconditional statements.

Example 3: Analyzing Biconditional Statements Consider the following statement: x = 3 if and only if x2 = 9. Is this a biconditional statement? The statement is biconditional because it contains the phrase “if and only if.” Is the statement true? Conditional statement: If x = 3, then x2 = 9. Converse: x2 = 9, then x = 3. The first part of the statement is true, but what about -3? That makes the second part of the statement false. So, the biconditional statement is false.

Example 4: Writing a Biconditional Statement Each of the following is true. Write the converse of each statement and decide whether the converse is true or false. If the converse is TRUE, then combine it with the original statement to form a true biconditional statement. If the statement is FALSE, then state a counterexample. #1 If two points lie in a plane, then the line containing them lies in the plane.

Example 4: Writing a Biconditional Statement (cont.) Converse: If a line containing two points lies in a plane, then the points lie in the plane. The converse is true as shown in the diagram on page 81. It can be combined with the original statement to form a true biconditional statement written below. Biconditional statement: Two points lie in a plane if and only if the line containing them lies in the plane.

Example 4: Writing a Biconditional Statement (cont.) #2 If a number ends in 0, then the number is divisible by 5. Converse: If a number is divisible by 5; then the number ends in 0. The converse isn’t true. What about 25? Knowing how to use true biconditional statements is an important tool for reasoning in Geometry. For instance, if you can write a true biconditional statement, then you can use the conditional statement or the converse to justify an argument.

Example 5: Writing a Postulate as a Biconditional Statement The second part of the Segment Addition Postulate is the converse of the first part. Combine the statements to form a true biconditional statement. If B lies between points A and C, then AB + BC = AC. Converse: If AB + BC = AC; then B lies between points A and C. Combine these statements to make a true biconditional statement Point B lies between points A and C if and only if AB + BC = AC.