formulo

A real seam, in the wrong language. Formulo genuinely is formo plus a diminutive: Latin -ula made small things out of big ones, so a formula was a little form. That is honest history, and it is not Esperanto grammar. The Esperanto diminutive is -et-, which would give formeto, and that word means a small shape and nothing to do with algebra. Formul- came over as one root. It is the same class as inspiri at 1,727, where in- plus spiri is perfectly visible and perfectly inert.

Worse: the pieces you can see are the wrong pieces. Read formulo with Esperanto eyes and the obvious parse is form- plus -ul-, the suffix for a person - the same -ul- that gives junulo a young person and, later in this same band, superulo a superior. That would make a formula into a shape-person, which is nonsense, and it is exactly the sort of nonsense a confident learner produces. When a borrowed root happens to end in -ul- or -et- or -ar-, check the dictionary before you decode.

What it means in use. A formulo is a fixed expression: the chemical one (la formulo de akvo estas H2O), the mathematical one, and the social one - a ĝentila formulo is the polite form of words you use to close a letter, which is the sense closest to Rome's original legal template. Formuli is the verb, to formulate or put into words, and formulaĵo is a set phrase that has stopped meaning much. The science article covers the register these words belong to.