Indirect Proof
AKA Proof by contradiction
Indirect proof is synonymous with proof by contradiction. A keyword signalling that you should consider indirect proof is the word 'not'. Usually, when you are asked to prove that a given statement is NOT true, you can use indirect proof by assuming the statement is true and arriving at a contridiction.The idea behind the indirect method is that if what you assumed creates a contradiction, the opposite of your initial assumption is the truth.
Example of an Indirect Proof
Prove that $$ \angle $$BDA is not a straight angle.
Statement 
Reason 
1) $$ \angle $$BDA is a straight angle 
1) (Assumpe opposite \) 
2) m$$ \angle $$BDA = 180° 
2) Definition of a striaght angle 
3)AD $$ \perp $$ BC 
3) Given 
4) $$ \angle $$BDA is a right angle 
4) definition of perpendicular lines 
5) m $$ \angle $$BDA = 90° 
5) definition of a right angle 
6) m $$ \angle $$BDA is not a straight angle 
contradiction of steps 2 and 5 
7 $$ \angle $$BDA 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 ($$ \angle $$bda is a straight angle) is false. Therefore, the opposite must be true: $$ \angle $$ is not a straight angle 
