From the Textbook: Discrete Mathematics and Its Applications (7th Edition). The course is Comp Sci, Discrete Structures.

7. Define E-expressions as follows: (a) 2, 3,4 are E-expressions These E-expressions are called atomic. (b) If x and y are E-expressions, so are (x + y) and (x * y). E-expressions are evaluated in the normal way. Show by induction that the value of every E-expression is at least 2n, where n is the number of atomic expressions. Hint: Let val(E) be the value of E and let #E be the number of atomic expressions in E