Lambda calculus is about ANONYMOUS functions.

(lambda x . expression)

means

a function with formal parameter "x" and body "expression".

Can we work with anonymous functions?

Reduce:
( lambda x . x * x * x ) ( 3 )
Reduce: ( lambda x . x * ( x + 1 ) / 2 ) ( ( lambda x . x * ( x + 1 ) / 2 ) ( 3 ) )
Reduce: ( lambda f . ( lambda x . f ( f ( x ) ) ) ( lambda x . x + 1 ) ( 3 )