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indirect proof

noun
  1. an argument for a proposition that shows its negation to be incompatible with a previously accepted or established premise.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for indirect proof

indirect proof

noun
  1. logic maths proof of a conclusion by showing its negation to be self-contradictory; reductio ad absurdumCompare direct (def. 17)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012