Ex-1.32

;a
(define (accumulate combiner null-value term a next b)
  (if (> a b)
      null-value
      (combiner (term a)
                (accumulate combiner null-value term
                            (next a) next b))))

;sum
(define (sum term a next b)
  (accumulate + 0 term a next b))
;product
(define (product term a next b)
  (accumulate * 1 term a next b))

;b
(define (accumulate combiner null-value term a next b)
  (define (iter a result)
    (if (> a b)
        result
        (iter (next a) (combiner result (term a)))))
  (iter a null-value)) 

Important thing to notice is that how we reached to accumulator, we wrote sum, product & identified a common pattern and abstracted it out in accumulate. Higher order procedure give more flexibility to apply this concept.