The Total solution for NCERT class 6-12
Given f(x) = |x|,
Now we have to provethat fof = f.
Consider(fof) (x) = f (f (x))
= f (|x|)
= ||x||
= |x|
= f (x)
So,
(fof) (x) = f (x), ∀x ∈ R
Hence, fof = f