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.
|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|