Proof by contradiction is a powerful tool.
Father of geometry
While Euclid extensively used proof by contradiction (reductio ad absurdum) throughout 'Elements', this specific phrasing is a modern summary of his method, not a direct quote.
c. 300 BCE (implied by method)
Found in 1 providers: grok
Cross Reference
1 source
"In any triangle, if one of the sides be produced, the exterior angle is equal to the two interior and opposite angles, and the three interior angles of the triangle are equal to two right angles."
Strange & Unusual"Trilateral figures are those contained by three straight lines, quadrilateral those contained by four, and multilateral those contained by more than four straight lines."
Shocking"When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right, and the straight line standing on the other is called a perpendicular …"
Strange & Unusual"No trace of Euclid's personality has survived."
Shocking"What advantage shall I get by learning these things?"
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