
 Proof techniques 
  A proof is a sequence of statements, each of which is an axiom, previously proved theorem, or is derived from previous statements in the sequence by means of a rule of inference. 
  A direct proof of a statement of the form A→B, begins with the assumptions encapsulated in A, and proceeds to construct a sequences of statements each of which is an axiom, previously proved theorem, or follows from previous statements by a rule of inference. 
  Proof by contradiction is also known as indirect proof, topdown proof, or goal directed proof. 
 cs.wwc.edu /KU/Logic/ProofTech.html (400 words) 
