Straight Angles

A  ______________________________  C                                                                     B

Straight angles have a measure of exactly 180 degrees.  They look exactly like a staight line.  Here are some examples:

A________________________________ C                                B

D _____________________________  F                              E

Theorem: If two angles are straight angles, then they are congruent.  Here is how we can prove this theorem: Given:  angle ABC is a straight             angle.             angle DEF is a straight             angle. Prove:  angle ABC is congruent             to angle DEF. Statements                                              Reasons 1.........................                                   1. Given 2. Measure of angle ABC is                      2. Straight angles have a measure 180 degrees.                                             of 180 degrees. 3.Measure of angle DEF is                       3. Same as step 2 180 degrees. 4. Angle ABC is congruent to                    4. If angles have the same angle DEF.                                               measure then they are congruent Note: You can assume straight angles from the diagram.