(f o g) (x) = ( f (g(x) ),

The domain of f _o g such that x is in the domain of g and that g(x) is the domain of f.

ex.

f(x) = x and g(x) = x + 2 find (f _o g) (x).
(f _o g) (x) = f (g(x))

= f ( x + 2 )
= ( x + 2 )²

The domain of g(x) = x + 2 is the set of all real numbers, and so is the domain of
f(x) = x² thus the domain of (f _o g) (x) is the set of real numbers.