How about formulae? Wikipedia: The informal use of the term formula in science refers to the general construct of a relationship between given quantities. The plural of formula can be spelled either as formulas or formulae (from the original Latin). Grammarist: Latin words appropriated by the English language usually form their plurals by the English method of adding an “s”, except in academic circles where the plural formulae is often the norm. And in this section on science the alternative "academic circles" formulae would certainly seem a reasonable alternative.
The word "effective" has several meanings in English, one of them is "efficacious". Did you try using "efficacious" and Duolingo didn't accept it? Then you could go ahead and report the problem, but "effective" is also correct.