Undefined Terms and Definitions 2-2A

Why do we need undefined terms?

What is a plane?

What is a biconditional statement?

Undefined terms

Name two undefined terms that we simply “understand”.

Point

Line

Third Undefined Term

Plane: a flat surface that extends infinitely. It has infinite length and width, but has not thickness.

A plane is named by a single capital letter.

P

Definition

Coplanar points:

points that lie in the same plane.

A

B

C

D

P

Noncoplanar points

Points that do not lie on the same plane.

AB

C

D

P

Q

Critical Attributes

Turn to page 109 in your book.

What Makes a Midpoint?

Turn to the Explore on page 110.

Definition of Biconditional Statement

When both a conditional and its converse are true, they are combined into one statement using the connector phrase “if and only if”.

“if and only if”

Definition

Point M is the midpoint of AB if and only if it divided AB into two congruent segments, AM and MB.

A BM

Both of the following statements are true.

• If M is the midpoint of AB, then it divides AB into two congruent segments.

• If a point M divided AB into two congruent segments, then it is the midpoint of AB.

A BM

Assignment 2-2A

Page 111, 1-6, 9-12, 19, 20