Logarithmic terms I've found a formula derivation in my international economics book, but I can't understand how it was derived. It says $ E P X = P^* IMP $ where E is the exchange rate, P is the level of national prices, X represents export, P* is the level of foreign prices, IMP represents import. Then it derives it as: $ e + p + x = p^* + imp $ saying that now it's expressed in terms of average annual variations. In a note, it says that this means that we're rewriting the first formula in terms of natural logs and differentiating it with respect to time. Please, could you give me a hint (or an explaination, if it's not too long) about the meaning of this derivation? Thank you in advance and sorry for my awkward English

In general:

$$\log(a b) = \log(b) + \log(b)$$ So in your case: $$\log(EPX) =\log(E)+\log(P)+\log(X)$$ Which your textbook wrote as: $$e+p+x$$