1) ∠is a straight angle ...........1) Assumpe opposite
2) m∠°.................... 2) Definition of a striaght angle
3)⊥ ̅ ....................................3) Given
4) ∠ is a right angle................. 4) definition of perpendicular lines
5) m ∠}°.................... 5) definition of a right angle
6) m ∠ is not a straight angle................ contradiction of steps 2 and 5
7) ∠ is not a straight angle..................... due to the contradiction between 2 and 5, we know that the assumption that WE INTRODUCED
in the first step (∠ is a straight angle) is .
Therefore, the opposite must be : ∠ is a straight angle
