The plural 's' at the end means there are more than one. If there are more than one there can be no single formula.
A formula or an expression.
No, it is not. A function can only have one output per input. (If it has more than one, it is still maths, but it cannot be called a "function". It would probably be called an equation or a formula etc...).
You can say Formulas or Formulae.
A person who can speak more than one language is called a polyglot.
well there is more than one!
Yes.
Yes it can.
you kinda answered your own question... if you have one, its pronounced fungus. If you are talking about more than one its fungi.
It can be called a formula.
Oxen *is* the plural -- more than one ox.
more than one lynx is call lynxes