The SUMSQ function syntax has the following arguments: Number1, number2, ... Number1 is required, subsequent numbers are optional. 1 to 255 arguments for which you want the sum of the squares. You can also use a single array or a reference to an array instead of arguments separated by commas.
THEY/THEIR (SINGULAR). Using the plural pronoun to refer to a single person of unspecified gender is an old and honorable pattern in English, not a newfangled bit of degeneracy or a politically correct plot to avoid sexism (though it often serves the latter purpose).
And another handful of nouns might seem to be singular in nature but take a plural form and always use a plural verb: My pants are torn. (Nowadays you will sometimes see this word as a singular pant [meaning one pair of pants] especially in clothing ads, but most writers would regard that as an affectation.)