meaning of f '(1/x)

What is the meaning of f '(1/x)?Does it mean(1) differentiate f(x) with respect to 1/x (this is what "Breakthrough Pure Maths Calculus" said), OR(2) find f ' (x) first (i.e. differentiate f (x) with respect to x first), and then put x into 1/x, OR(3) find f(1/x) first, and then differentiate the resultant expression with respect to x? I ask this question because I think (2) should be the correct one (Consider the meaning of f '(2), you WON'T differentiate f(x) with respect to 2! It will always equal to zero!) But on the other hand, I don't think an A-level textbook will make such a mistake? Thanks for answering.