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Answer to Question equation-8

Here is an evaluation using the following equations. 
(sum.1)  sum([])  = 0
(sum.2)  sum(h:t) = h + sum(t)
Evaluation is as follows. 
sum([3,5,7]) = sum(3:[5,7])
             = 3 + sum([5,7])              by (sum.2)
             = 3 + sum(5:[7])
             = 3 + (5 + sum([7]))          by (sum.2)
             = 3 + (5 + sum(7:[]))
             = 3 + (5 + (7 + sum([])))     by (sum.2)
             = 3 + (5 + (7 + 0))           by (sum.1)
             = 15                          by arithmetic