Which Was To Be Proved

"Quod erat demonstrandum" — which was to be proved — is the Latin translation of the Greek phrase Euclid used to conclude each geometric proof in the Elements (c. 300 BC), signaling that the stated proposition had been derived from axioms and previously established theorems. The formula became the standard closing of mathematical proofs in European and Islamic scholarship for centuries. Its abbreviated form, QED, remains in use in formal mathematics today.