Top//Answer-29-6

Answer to Question 29-6

Here is an evaluation using the following equations. 
(sum.1) sum([])  = 0
(sum.2) sum(L) = head(L) + sum(tail(t))   (when L is not empty)
Evaluation is as follows. 
sum([3,5,7])
   = 3 + sum([5,7])              by (sum.2)
   = 3 + (5 + sum([7]))          by (sum.2)
   = 3 + (5 + (7 + sum([])))     by (sum.2)
   = 3 + (5 + (7 + 0))           by (sum.1)
   = 15                          by arithmetic