Isaac Newton was born on this day in 1643

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ERAT (ER-awt)

Quod Erat Demonstrandum: ‘that which was to be demonstrated’

Common clues: Part of QED; Latin 1 verb conjugation; QED middle; Quod ____ demonstrandum

Crossword puzzle frequency: 6 times a year

Frequency in English language: (QED) 49166/86800

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Q.E.D. is an abbreviation of the Latin phrase "quod erat demonstrandum" (literally, "which was to be demonstrated"). In simple terms, its use is to indicate that something has been definitively proven.

Q.E.D. may be written at the end of mathematical proofs to show that the result required for the proof to be complete has been obtained. It is not seen as frequently now as it once was, since formal geometry is less commonly taught as a separate subject.

The term is also used both formally and informally in a wide variety of disciplines, as well as in everyday conversation in many parts of the English-speaking world.

A page from Isaac Newton’s notebook

Q.E.D. is a translation into Latin of the original Greek (hoper edei deixai) which was used by many early mathematicians including Euclid and Archimedes. Benedict De Spinoza also makes extensive use of the abbreviation Q.E.D. in his various works.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Q.E.D."