classically correct plural of formula; also see -a (1).
Men who try to speak what they believe, are naked men fighting men quilted sevenfold in formulae. [Charles Kingsley, "Letters," 1861]
Entries linking to formulae
formula n.
1630s, "words used in a ceremony or ritual" (earlier as a Latin word in English), from Latin formula "form, draft, contract, regulation;" in law, "a rule, method;" literally "small form," diminutive of forma "form" (see form (n.)). Modern sense is colored by Carlyle's use (1837) of the word in a sense of "rule slavishly followed without understanding" [OED]. From 1706 as "a prescription, a recipe;" mathematical use is from 1796; chemistry sense is from 1842. In motor racing, "class or specification of a car" (usually by engine size), 1927.
-a 1
word-forming element which in English is characteristic of fem. nouns and adjectives of Latin or Greek origin (such as idea, coma, mania, basilica, arena, formula, nebula). From Latin -a (plural -ae) and Greek -a, -ē (plural -ai, Latinized as -ae). The Latin suffix also became Italian -a (plural -e), Spanish -a (plural -as). It is represented in Old English by -u, -e, but even then the suffix was fading and by the time of modern English was totally lost or swallowed into silent final -e-.
It also appears in Romanic words from Latin that have been borrowed into English, such as opera, plaza, armada. It figures in scientific names coined in Modern Latin (amoeba, soda, magnolia, etc.) and is common in geographical names formed according to Latin or Greek models (Asia, Africa, America, Arabia, Florida, etc.)
In English it marks sex only in personal names (Julia, Maria, Alberta) and in a few words from Italian or Spanish where a corresponding male form also is in use (donna, senora).