# GCSE maths self-inverse functions

Discussion in 'Mathematics' started by gainly, Apr 29, 2019.

1. The following question is in Edexcel GCSE Mock Paper 2 Autumn 2018. It seems to assume they should know that if ff(x) = x the function is self-inverse. Of course they could work out the inverse function, but as the question says "write down" and is only one mark this is clearly not what was intended.

There is no mention of self-inverse functions in the specification. Are they meant to know this? It seems a bit unreasonable to me.

20. f(x) = (1-x)/(1+x)

(a) Show that ff(x) = x (3)

(b) Hence, write down f ^–1(x) (1)

(Total for Question 20 is 4 marks)

2. Although self-inverse functions are not explicitly mentioned in the specification the two Higher tier textbooks that I have both have exercise questions that would involve the topic, so possibly it's expected that it would be discussed in a lesson?

e.g My AQA book has the following question :-

Find the inverse of these functions.

a. f(x) = 1/x
b. g(x) = 2 - x